To extend your stay in Japan with your current visa (status of residence), you need to apply for “Extension of Periods of Stay” before your current visa is expired.
Mid-long term residents may be submitted from around 3 months before the expiration date of the period of stay.
So what happens if my visa expiry day comes while my application for “Extension of Period of Stay” is still in process?
If you apply for an extension before your visa expiry and no decision has been made by the expiration date, generally speaking, you can stay in Japan under the current visa until a decision is made by the immigration official.
Otherwise, you are allowed to stay in Japan for another 2 months from the original expiration date.
In general, immigration officials will make effort to conclude an application process within 2 months extension period. However, if an applicant fails to visit the regional immigration bureau immediate after a notification letter from them arrives (i.e. a week), he/she will become an overstayer no matter what his/her application outcome is.