1 is located at the bottom dead center is 180 u00b0 of rotation of the main shaft 5 as shown in fig4 when the rotation of the main shaft 5 is 0 u00b0 the groove of the looper drive cam 6 is at the maximum slant position of u03b8 1 and the lower looper 2 is located at the rightmost end when the rotational position of the main shaft 5 is at 180 u00b0 the groove of the looper drive cam 6 is at the maximum slant position of u03b8 1 and the lower looper 2 is located at the left most end the phase of the upper looper 3 is offset by about 35 u00b0 when the rotational position of the main shaft 5 is at 35 u00b0 the groove of the looper drive cam 7 is at the maximum slant position of u03b8 2 and at this time the upper looper 3 is located at the top dead center as shown in fig5 when the rotational position of the main shaft 5 is 180 u00b0 35 u00b0 the groove of the looper drive cam 7 is at the position of u03b8 1 and at this time the upper looper 3 is located at the bottom dead center as shown in fig4 incidentally in the foregoing embodiment the upper looper 3 operates with an offset angle of about 35 u00b0 relative to the lower looper 2 it is apparent that the invention is not limited to this offset angle and it is possible to carry out the sewing operation normally.