In the case of looper thread breakage, the looper thread twining prevention mechanism is activated to trim the looper thread before the thread twines on the looper thread cam. In this way, the looper thread twining prevention mechanism prevents the looper thread from twining on the looper thread cam. In addition, the looper thread cam, which has been incorporated in the sewing machine, has been changed so that it is mounted outside of the sewing machine. The externally-mounted looper thread cam promises improved maintainability.