inner rotary hook 9 is pivotably supported on the outer rotary hook 8 the driver 10 is fixed to the distal end of a rotary hook shaft 11 that is rotatably supported by the outer rotary hook 8 bobbin case 12 having accommodated therein a bobbin on which is wound a lower thread is removably set in the semi rotary hook 7 transmission mechanism 13 for pivoting the driver 10 of the semi rotary hook 7 in response to rotation in one direction of the cylindrical grooved cam 5 is provided in front of the cylindrical grooved cam 5 fig3 is an exploded perspective view of the transmission mechanism 13 and fig4 is a front sectional view of the transmission mechanism 13 as clearly seen from these figures the driver 10 is fixed to the distal end of the rotary hook shaft 11 a driven gear a 2 is mounted coaxially on the rear end of the rotary hook shaft 11 and held in meshing engagement with a sector driving gear a 1 fixedly mounted on a support shaft 18 interlocking arm 200 is fixed to an axial end portion of the support shaft 18 in contact with the rear surface of the sector driving gear a 1 and a roller 20 which is a cam follower is supported on a free end portion of the interlocking arm 200 the cam follower 20 is fitted in a groove portion 5 a of the cylindrical grooved cam 5 thus as the cylindrical grooved cam 5 is driven to rotate in the one direction together with the drive shaft 3 the.