database log grows in mirroring full safe configuration

hi, i'm gabriele DBA of samu.it.
sorry for my english ... i'm italian and i write not so good.
i've a problem with the log file in a db mirroring (full safe with witness) configuration. it grows and it becomes very big in a few time. the only way i've found to reduce the size of the file is to stop mirroring, change the recovery mode in simple and shrink. i backup the database every 12 hours but log file grows the same.
have u got any other suggestion?
thank u very much for your answer.

Have you got scheduled log backups? You need these for a database which is being mirrored (the principal), just like any other database.

even if we have a log backup every 1 hour, the log file doesn't reduce.
have u got any other suggestion?

have you checked for open transactions? (use dbcc opentran(databasename) for this). Are you running dbcc indexdefrag on the database?

no. have u got any reference about?

HI Samu,
I had similar issue. What I did was; I took full backup and then Log backup subsequently, shrink the database. This truncated the log and allocated free space. Can you try this?