I'm not sure how easy it is in Japan but in Singapore, It just took me 10-15 mins to file my tax. 1. Login to IRAS(tax bureau) portal using the national ID (for

I'm not sure how easy it is in Japan but in Singapore, It just took me 10-15 mins to file my tax. 1. Login to IRAS(tax bureau) portal using the national ID (for me as foreigner, I used my employment pass ID) 2. Open the page for filing income tax 3. I entered total annual salary and allowances 4. I have donated to a government-recognized charity and this is automatically reflected on the form. It seems that the charity uploads donation infos to IRAS 5. Check if the tax relief for having a child is there 6. Click submit 7. Review and re-submit You also have an option to do installment, deducting from your bank account monthly. I don't have any other source of income so it will be different for other people.