If you earned more than ￥20 million from sources of salary in 2020, there are two things to watch out for.
First, you will have to file your own individual income tax return. It is due by April 15th of 2021.
Second, the Report of Assets and Liabilities. If you owned more than ￥300 million worth of assets, or more than ￥100 million worth of assets such as securities or unsettled derivatives etc., you will have to file this report together with your individual income tax return.