An over-lock sewing machine executing an over-edge chain stitch and a double-chain stitch includes a sewing machine motor for driving a needle bar so that the needle bar is moved upward and downward, a sewing needle mounted on a lower end of the needle bar, an upper looper and a lower looper forming stitches in cooperation with the sewing needle, and a stepping motor for driving the lower looper so that the lower looper is rocked.