6 in a position corresponding to the recess 20 and by engagement between the recess 20 and the projection 21 the thread guide supporting member 17 is held in a perpendicular state at an end of the recess 20 in order to hold the thread guide 14 in a feeding position of the upper decorative thread in the case of the flat stitch and the over lock stitch in which no upper decorative stitch thread is fed when a rotary operation is made by 90 degrees in the arrow marked direction in fig1 with a switching operation knob 22 provided on the lateral side of the thread guide supporting member 17 the projection 21 on the frame 6 side is engaged with the other end of the recess 20 on the thread guide cam 18 side to hold the thread guide 14 in the obviating position shown by the dashed line in the same figure both side ends of the recess 20 are formed optionally in a deeper style so that they can be engaged by the projection to stop in a manner of click motion needless to say engagement and release between the recess 20 and the projection 21 are elastically performed against the bines of the spring 19 which forces the thread guide.