副業向け

副業の経費と確定申告ガイド【2026年版・会社バレ対策】

会社にバレずに副業収入の経費管理と確定申告を行う方法を解説。青色申告の選択肢、住民税の普通徴収切替、20万円ルール、節税のポイントまでまとめました。

2026年1月1日

|Source:国税庁

一般的な参考情報です。個別の税務判断は税理士にご相談ください。最終確認日: 2026年1月1日 · 出典: 国税庁

会社員として働きながら副業をしている方向けの経費管理ガイドです。正しく経費を計上することで節税効果を得られますが、同時に会社にバレないための注意点も押さえておきましょう。

確定申告が必要なライン

給与所得者の場合、副業による所得が年間20万円を超えると確定申告が必要です。ただし、20万円以下でも住民税の申告は必要です。

会社にバレないためのポイント

Choose "Direct Payment" for Residence Tax重要

By selecting "Self Payment" in the residence tax section of your tax return, your side income tax will not be notified to your employer.

Check Company Regulations

Check if side jobs are prohibited. If permission is required, consider applying formally.

Avoid Competing with Main Job

Side jobs competing with your main job may violate regulations. Also avoid using main job information.

Understand the ¥200,000 Rule重要

Even if side income is under ¥200,000/year, residence tax filing is required. File with your local municipality even if income tax filing is not needed.

副業で認められる経費

Computer & Equipment

経費にできる

+Computer/tablet (business dedicated)
+Monitor and keyboard
+Camera and microphone (for streaming/recording)
+Software and subscriptions

経費にできない

-Computer shared with personal use (full amount)
-Gaming/entertainment equipment

ポイント: Prorate by business use if shared. Under ¥100k is immediate expense, Blue Return allows up to ¥300k.

Workspace

経費にできる

+Rent (business use area)
+Electricity (business portion)
+Internet (business portion)
+Coworking space fees

経費にできない

-Mortgage principal
-Personal portion of utilities

ポイント: If working from home, prorate by floor area. Typically 20-40% is common.

Communication

経費にできる

+Mobile phone charges (business portion)
+Video conferencing tools (Zoom, etc.)
+Cloud storage
+Domain and hosting fees

経費にできない

-Personal call charges
-Entertainment subscriptions

ポイント: If shared between business and personal, prorate by call logs or usage time. Typically 30-50%.

Learning & Skills

経費にできる

+Business-related online courses
+Professional and technical books
+Business-related certification costs
+Seminar and conference fees

経費にできない

-Hobby classes
-Unrelated certifications
-Entertainment books

ポイント: Relevance to your side job is key. Fully deductible if you can explain the business connection.

確定申告の準備チェックリスト

Organize Your Income

Get annual payment statements from each platform
Check bank account deposit records
Collect withholding tax slips (if any)

Calculate Your Expenses

Organize receipts by expense category
Record calculation basis for prorated expenses
Also use credit card statements

Prepare Tax Return

Use the NTA tax return preparation site
Combine salary income and business (miscellaneous) income
Select "Self Payment" for residence tax

File and Pay

Online filing via e-Tax is recommended
Deadline is March 15 (income tax)
Pay by bank transfer or credit card before deadline

住民税の選択を忘れずに

確定申告書の「住民税・事業税に関する事項」で「自分で納付」を選択しないと、会社に通知されます。

よくある質問

The most common cause is residence tax. If you leave the payment method as "Special Collection (payroll deduction)" when filing, your employer receives notification of residence tax including side income, which payroll staff may notice. By selecting "Direct Payment (self payment)", you pay side income tax directly and it is not notified to your employer.

Income tax filing is not required, but residence tax filing is. If you neglect residence tax filing, you may face additional taxes later. Either file directly with your municipality, or file income tax return (which automatically files residence tax). Also, if you want medical expense or furusato deductions, income tax filing is required even under ¥200,000.

Yes, if you work on your side job at home, you can claim part of your rent as an expense. Calculate by floor area. For example, if using 6㎡ for work in a 30㎡ room, 20% of rent can be expensed. Keep records of your calculation basis.

It depends on the scale of your side job. If you have continuous income at a certain level (roughly ¥1M+ annually), Blue Return is beneficial. You get a ¥650,000 special deduction and can carry forward losses. However, it requires double-entry bookkeeping. For small side income, White Return is fine.

経費にできる？チェッカー

その支出が経費にできるか即座にチェック。勘定科目や控除率も一目でわかる無料ツール。