sideline business and part time jobs

Is it okay for a gaijin to have a sideline business (like selling little stuffs) or having a part time job (like on-call english tutor) while working as a full time employee in japan? I mean will Japanese employers be okay with it and will have nothing against it?

Actually i dont have a concrete plan yet. I just remember that i used to sell when i was a student

Depends on the terms of your contract.

What if nothing in the contract specified that kind of setup? I am more concerned if they considered it ethical or not in japan and if its common for salary men in general to have extra source of income

Little sidelines like selling stuff on auction probably wouldn't be a problem, but part time jobs may be an issue even if they are not expressly prohibited in your contract. Also note that part time jobs would require you to get prior permission from immigration, and they could be difficult to hide especially with the new mynumber system that is set to debut shortly.