Analyzing the Impact of the 2026 Tax Reform '10-Year Rule': Retirement Benefit & iDeCo Optimization Simulator Releases Data

Mycat Inc. (Headquarters: Meguro-ku, Tokyo) has released data analyzing how the '10-year rule' introduced in the 2026 tax reform affects recipients of retirement benefits and iDeCo through its 'Retirement Benefit & iDeCo Optimization Simulator' (https://taisyoku.xyz).

What is the '10-Year Rule'?

Under the 2026 tax reform, the interval required to avoid the overlapping application of retirement income deductions will be extended from the current 5 years to 10 years. This rule directly impacts tax calculations when receiving both retirement benefits and iDeCo.

Previously, if one received iDeCo as a lump sum and then received retirement benefits 5 years later, they could fully utilize the retirement income deduction for both. However, with the interval extending to 10 years, there will be cases where this strategy is no longer effective.

Scale of Impact

According to statistics from the National Pension Fund Association, the number of iDeCo subscribers exceeded 3.5 million as of 2025. A significant number of these individuals are estimated to be using corporate retirement benefit plans concurrently.

According to the Ministry of Health, Labour and Welfare's 'General Survey on Working Conditions,' 74.9% of companies have retirement benefit systems. For those who are enrolled in iDeCo while working at companies with retirement benefit plans, the 10-year rule necessitates a review of their withdrawal strategies.

Simulation Result Trends

Calculations using the simulator confirmed the following trends:

• Case: 30 years of service, 20 million yen in retirement benefits, 5 million yen in iDeCo: Depending on the combination of withdrawal methods and timing, the difference in tax amounts ranges from approximately 500,000 to 1.5 million yen.
• Case: 20 years of service, 10 million yen in retirement benefits, 3 million yen in iDeCo: Due to the application of the 10-year rule, net income may decrease compared to the traditional '5-year gap' strategy.
• Combined Pattern (Lump sum + Pension): Depending on the allocation ratio, differences of several hundred thousand yen in tax can occur.

The difference in net income based on withdrawal methods tends to increase as the amounts of retirement benefits and iDeCo assets grow.

Utilizing the Free Simulator

The simulator allows users to check comparison results for all patterns simply by entering individual conditions. It is completely free to use and does not require member registration.

Data Publication Page: https://taisyoku.xyz/report
Service URL: https://taisyoku.xyz

※ This service is not a substitute for tax advice. Please consult a tax accountant for specific decisions.

■ Company Overview
Company Name: Mycat Inc.
Established: February 5, 2025
Address: 2-7-22 Mita, Meguro-ku, Tokyo
Business Description: Planning, development, and operation of AI-powered services for SMEs and individuals
Corporate Website: https://mycat.business
Contact: [email protected]