To prevent twist of a part of a loop material between its part which is sewn on a sewn product body and its part which is held by a clamping means during cloth feeding movements of a sewing machine and thereby to sew belt loops in a uniform shape and of high quality by the sewing machine.

A control means 17 controls driving of a Y-direction moving means 12, a Z-direction moving means 13 or the like so that the clamping means 3 may move in the direction in which the part of the loop material 20 between its part which is held by the clamping means 3 and its part which is pressed by a pressure foot 23 loosens after the sewn product body 21 and the loop material 20 are pressed by the pressure foot 23 of the sewing machine 2 and before the sewing machine 2 sews the loop material 20 on the sewn product body 21 as it feeds the sewn product body 21 in accordance with the shape to be sewn.