PURPOSE: To shorten the time required for the synchronous control in response to a change of the output side speed by providing a circuit measuring the input side speed, output side speed, and position of a looper and calculating the optimum speed difference between the input side and the output side and the optimum speed at the input side.

CONSTITUTION: Results measured by an input side speed measuring circuit 13, a center speed measuring circuit 12, and a looper position measuring circuit 14 are input to an optimum speed difference calculating circuit 16, and the speed difference between the input side and center (output side) required to guide to the looper synchronous position is calculated. This calculated result and the measured result by the center speed measuring circuit 12 are input to an input side speed calculating circuit 17, and this calculated result is input to an input side speed reference circuit 11. As a result, the synchronous control of a looper can be performed in a very short predetermined time.