When engaging in part-time work in Japan, you must abide by the conditions stated below. Failure to do so may result in punishment and even deportation.
1. Apply for “permission to engage in activities other than that permitted under the status of residence previously granted” at the nearest immigration services bureau. The application form is available at the Office of International Affairs. Please come to the Office of International Affairs with your passport and resident card.
2. The part-time work does not affect your studies.
3. The earned income is meant to supplement your academic cost and necessary expenses and not for saving or for remittance overseas.
4. The part-time work engaged in is not in adult entertainment businesses. Note: Prohibited by law.
5. Within 28 hours a week (up to eight hours a day during the long school holidays)
6. The part-time work is done while you retain student status in an educational institution.

Difference between part-time jobs and internships
Part-time job: Work in which you are paid money for your labor.
Internship: Work experience in which you are given a work opportunity that will be useful for your career plans, etc. Internships are often unpaid.

【IMPORTANT】
Every year, there are international students who are forced to leave Japan as they cannot extend their period of authorized stay in the country due to their low school-attendance rates because they work part-time and neglect their studies. You have to be very careful not to let this situation happen.