PURPOSE: To facilitate the zigzag control and the exchange of an endless belt by providing regulation means to be fitted in guide members provided on both sides of the belt and retreated temporarily.

CONSTITUTION: On both sides of a photo-sensitive belt 4, perforations 29 are perforated, and sprocket wheels 31 and 32 are rotatably provided on a support shaft 33 on the outsides of the belt for fitting in the perforations 29. In the center part of the support shaft 33 of the sprocket wheels 31 and 32, levers 34 and 35 are provided to be rotated round a shaft 36. The shaft 36 is fixed on the outside device 1, and on the levers 34 and 35, springs 37 are fitted to keep the sprocket wheels 31 and 32 in touch with a roller 7. This construction permits to facilitate the control of the zigzag movement and the exchange of the belt.