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' in the 2026 tax reform will affect recipients of retirement benefits and iDeCo, using 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 the retirement income deduction 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 you received your retirement benefit 5 years after taking your iDeCo as a lump sum, you could fully utilize the retirement income deduction for both. However, with the interval extending to 10 years, this strategy will no longer be effective in many cases.

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 portion of these individuals are estimated to be using corporate retirement benefit plans simultaneously.

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

Simulation Trends

Calculations from the simulator confirmed the following trends:

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

The difference in net income based on withdrawal methods tends to increase as the retirement benefit amount and iDeCo asset size 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 Release 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: Planning, development, and operation of AI-based services for SMEs and individuals
Corporate Website: https://mycat.business
Contact: info@mycat.lo...