If a foreigner with a family stay visa wants to work part-time, he / she can work by applying for permission to work outside the status of qualification.
Generally, it is said that permission is not required for one-off event support or sales of goods or services such as Mercari.
There are some side job options by not employed in Japan.
If you are teacher, there are some options as below
Since it is basically a Japanese-based service, you need to have reading comprehension and responsiveness in Japanese. The job of teaching in Japanese requires a high level of Japanese proficiency, but the job of teaching your mother language is ideal.
Street Academy- A market that connects what you want to teach and what you want to learn
Coconala -Japanese largest skill market -Buy and sell everyone's strengths
Time Ticketー Buy and sell personal time in 30 minute increments
If you want to sell goods, there are some options below
Since it is basically a Japanese-based service, you need to have reading comprehension and responsiveness in Japanese. However, since it is a product sale, it is not necessary to have that level of Japanese ability to provide the service itself.
Mercari-Japanese giant goods Market on App
Rakuma-Japanese goods Market run by Rakuten
Yahoo Auction- Japanese Auction run by Yahoo！
If you want to do simply hourly gig work, there are some options below
Since it is based on Japanese, reading comprehension and responsiveness in Japanese are required.
Lancers-Japanese largest crowd sourcing
Crowdworks－Japanese largest crowd sourcing