A cutter adapter includes a holder having a receiving portion for inserting a cutter therein; a sleeve sleeved onto the holder and having an annular bevel; a lock pin inserted into the holder and having an active end and a passive end, the passive end being squeezed by the annular bevel when the sleeve is moved, the active end being inserted into the cutter when the passive end is squeezed; and a biasing pin located in the receiving portion and pushed by a spring mounted in the receiving portion. In this way, when intending to mount the cutter to the cutter adapter, the user only needs to insert the cutter into the receiving portion to force the biasing pin to disengage from the lock pin, the lock pin being forced by the annular bevel to insert the active end into the receiving portion to hold the cutter.