To detect the overload of a motor accurately.

A shutter curtain 12 formed by foldably connecting a large number of slats 14 by rotating a drum 40 in one direction and the other direction by a motor 20 is wound on the drum 40 and delivered from the drum 40 respectively. A controller, by which pulse waves synchronized with the rotation of the motor 20 are generated at that time, the period of the pulse waves is measured at all times, a past period and the current period are compared, and the rotational control of the motor is changed when the difference of both periods exceeds a fixed value, is installed.