To increase the number of sectors whose breakage is detected by a monitoring element by constituting a front sector group so that it may be shifted while keeping a state where the monitoring element is shielded from a state where a 1st sector independently shields the monitoring element to a state where a 2nd sector independently shields the monitoring element by the traveling of the front sector group.

This front sector group 38 is composed of 1st and 2nd sectors 7 and 6 starting exposure by traveling to a position where they retreat to the outside of a photographic image plane from the state where the photographic image plane is covered. Namely, the sector group 38 is positioned in a gap between the monitoring element 19 and a reflector 18 in the state where the photographic image plane is covered, and is provided with the 1st sector 7 independently shielding the element 19 and the 2nd sector 6 passing the gap between the element 19 and reflector 18 continuously to the 1st sector 7 while the sector group 38 travels. The sector group 38 is constituted so that it may bet shifted while keeping the state where the element 19 is shielded from the state where the 1st sector 7 independently shields the element 19 to the state where the 2nd blade 6 independently shields the element 19 by the traveling of the sector group 38.