Is MySQL Server usage for small business is legal?

Today I'm using MS Access database shared with 2 computers (one holds it and share with the other). I'm about to expand and use extra computers. For this reason, I'm considering use of SQL server. One of my consideration is MySQL server. I'm about to use that for installing this Software on a server which will share it's content simultaneously with several computers (3-6) in the office. This MySQL server will be my backend and my front end will be MS Access. Is this kind of use is legally free? or must be paid for this kind of usage?

Thanks!