Japan Fixed Overtime Trap: How to Spot the ¥1,000,000/Year Hidden Pay Cut

Information current as of April 2026 based on MHLW precedent database on unpaid wages, MHLW fixed overtime disclosure guidance, MHLW startup labor Q&A, and Supreme Court decisions 医療法人康心会事件 (最判平29.7.7) and 日本ケミカル事件 (最判平30.7.19).

Fixed overtime is legal in Japan. It's also the clause where foreign employees lose the most money without realizing it. This article explains how to calculate what you're actually being paid, the three Supreme Court conditions that make a 固定残業代 clause valid, what an illegal clause looks like, and what to negotiate before you sign.

What 固定残業代 actually does

Under 労基法37条, an employer must pay 125%, 135%, or 150% of your hourly wage for various categories of overtime. That calculation is done monthly based on actual hours worked.

固定残業代 is a prepayment system: the employer includes a flat monthly amount in your salary to cover a fixed number of overtime hours. If your real overtime comes in below that number, you keep the extra as part of your take-home pay. If it exceeds the number, the employer must pay the difference.

That's the legal structure. In practice, it works three ways:

Company behavior	Legality
Pays 固定残業代 + difference when OT exceeds fixed hours	Legal
Pays 固定残業代 flat, never pays extra regardless of hours	Illegal (労基法37条 violation)
Treats 固定残業代 as part of "base salary," not separately itemized	Invalid clause — base becomes ALL of it for OT purposes

The three Supreme Court requirements

Two landmark cases define when a 固定残業代 clause is actually enforceable:

Requirement 1: 明確区分性 (Clear separation)

The basic wage and the fixed-overtime portion must be clearly separated on both the employment contract and each monthly payslip. "Salary ¥400,000" with no breakdown fails this test. Valid format:

• 基本給: ¥300,000
• 固定残業手当 (月30時間分): ¥70,313
• 合計: ¥370,313

If the clause fails 明確区分性, courts treat the entire amount as base salary — meaning overtime is calculated against the full ¥370,313, not ¥300,000.

Requirement 2: 対価性 (Genuine overtime compensation)

The fixed portion must actually compensate overtime work — not be a disguised base allowance with a misleading label. If the "fixed overtime" is the same yen amount regardless of hours worked (including when OT is zero) AND the contract labels it ambiguously (e.g., "業務手当"), courts may find no 対価性 and strike down the clause.

Requirement 3: 差額支払 (Paying the difference)

If your actual overtime under 労基法37条 calculation exceeds the fixed portion in any given month, the employer must pay the difference. This must be either explicit in the contract or consistently practiced in payroll. If the contract is silent on this point, the clause is vulnerable to challenge.

Precedent: 日本ケミカル事件 (最判平30.7.19) struck down a fixed-overtime clause where all three requirements were ambiguous, awarding the employee full retroactive overtime plus the fixed "overtime allowance" on top.

How to calculate your real base hourly

Every 固定残業代 clause should be reverse-engineered before you sign. Here's the math:

Assume a ¥4,800,000/year offer broken down as:

• 月給 ¥350,000
• 内訳: 基本給 ¥260,000 + 固定残業手当 ¥90,000 (45時間分)
• 賞与 ¥600,000/年 (2ヶ月分)

Monthly scheduled hours at typical Japanese company: ~163 hours (based on 40 hours × 52 weeks ÷ 12 months + holidays).

1. Real hourly wage for OT calculation = ¥260,000 ÷ 163 = ¥1,595/hour
2. Required OT rate = ¥1,595 × 1.25 = ¥1,994/hour
3. 45h × ¥1,994 = ¥89,730 → matches the ¥90,000 fixed allowance (legitimate)
4. If you work only 20 hours of OT, you pocket the extra ¥49,858 — that's the worker's side of the deal
5. If you work 60 hours of OT, employer owes (60-45) × ¥1,994 = ¥29,910 extra

The headline ¥4.8M sounds like a ¥2,300+/hour base. The real base is ¥1,595/hour. That's a 30% difference. Foreign hires comparing offers in USD without running this math routinely overpay in tax planning or underprice themselves against market.

Red-flag fixed-OT patterns

45 hours/month

This matches the 36協定 monthly maximum without special-measure clauses. A company writing 45h into every new contract is signaling they expect you to hit that cap. Combined with minimal 残業禁止 culture in Japanese workplaces, this usually means you will hit 45h most months.

"固定残業代を含む" without separate yen amount

Clauses like "月給には固定残業代を含む" with no separated yen amount or hours fail 明確区分性 instantly. This is the most common invalid clause in small-company contracts — including many aimed at foreigners.

"Includes" 80 hours or more

Courts have struck down clauses with 60+ hour fixed OT as effectively forcing overtime far beyond 過労死 thresholds (80h/month is the brain/heart disease line in 労災 decisions). Anything this high signals an employer already defending against labor suits.

No difference-payment clause

If the contract doesn't say what happens when you exceed the fixed hours, the employer is counting on you not asking. Ask explicitly in writing before signing: "If my actual overtime exceeds the fixed 30 hours in a given month, the difference will be paid per 労基法37条. Please confirm."

Negotiating a better deal

Once you understand the breakdown, three moves work:

1. Reduce fixed hours, raise base — "Move 15 hours of fixed OT into base salary" preserves total comp on a normal month and gives you a higher real hourly rate.
2. Eliminate fixed OT entirely — Replace with a flat base that matches the previous total at zero OT. If OT is rare in your role, this is strictly better for you.
3. Cap fixed OT at 20 hours — The standard for knowledge-worker roles that don't expect routine overtime. Most Japanese employers will accept this for senior hires.

Opening line: "The fixed overtime portion of 45 hours seems high for this role. Could we restructure the breakdown to ¥X base + ¥Y fixed OT for 20 hours, keeping total compensation the same?"

What to do if you already signed a bad clause

If your current contract has an invalid 固定残業代 clause (fails any of the three Supreme Court requirements), you may be entitled to retroactive overtime pay. The statute of limitations under 労基法 for unpaid wage claims is 3 years (raised from 2 years in April 2020).

Steps:

1. Gather payslips for the last 3 years
2. Calculate overtime against the full claimed salary (not just the base portion) if 明確区分性 fails
3. Request written clarification from HR; their reply is evidence
4. File at the 労働基準監督署 (free) or hire a 弁護士 specializing in 未払残業代

Deep dive on recovering unpaid overtime: How to Recover Unpaid Wages in Japan Before 3 Years Run Out. For an overall view of Japanese employment contracts: Japan Job Contract Guide for Foreigners.

The bottom line

Fixed overtime is not inherently bad — it simplifies accounting and can favor workers whose actual OT comes in under the fixed amount. But it requires three Supreme Court-mandated conditions (clear separation, genuine compensation, difference payment) to be enforceable. The clauses you should reject: buried breakdowns, 45+ hour fixed portions, missing difference-payment language. Run the math before you sign. Ten minutes with a calculator saves years of underpayment.

Related: Overtime Calculation in Japan: Fixed Overtime Traps and How to Check Your Pay covers payslip-level verification.