face 5 and retaining member 31 the follower 34 is rotated angularly in the direction of arrow 37 by means of the eccentric cam 16b as a result the contacting face 5 abuts against the contacting face 32 therefore the needle thread loop may pass smoothly between the contacting face 5 and the contacting face 32 of the retaining member 31 after the passing of the needle thread loop the follower 34 is rotated in a direction opposite to the direction of arrow 37 and the contacting face 5 moves from the contacting face 32 along communicating surface 38 to again abut the contacting face 10b of the retaining member thereby returning to the original position the communicating surface 38 smoothly connects the contacting face 10b and the contacting face 32 which permits smooth movement of the bobbin case 3 in the direction 7 and the reverse direction at the time of angular movement of the retaining member 31 in the embodiment in fig3 retaining member 31 is driven so as to be pivoted reciprocally but it is also possible to provide another embodiment by continously rotating a retaining member having a continuous contacting face in the peripheral direction contacting with the contacting face 5 forming the contacting plane 32 for slipping out the needle thread.