The hook
“Year-end adjustment makes many employees feel finished, but the NTA does not treat every salary earner as done.”
Japan's NTA explains that most wage earners finish through year-end adjustment, but it also lists cases where a final return is still required. Two of the most common triggers are employment income above 20 million yen or side income above the 200,000-yen threshold. So the question is not only whether payroll adjusted tax. It is whether other facts reopened the filing job.
The takeaway
In Japan, year-end adjustment solves many salary cases, but it does not erase side-income filing rules.