PURPOSE: To surely detect whether an aperture is a pin hole or a perforation, and to improve accuracy for detecting the perforation.

CONSTITUTION: The perforation 8 is photoelectrically detected by providing a light projector 27 and a light receiver 31 on opposed positions putting a film housing sheet 10 between them on the carrying path of the sheet 10. A time from the detection of the rising edge of the aperture constituted of the perforation 8 and the pin hole to the detection of falling edge of the aperture is detected as aperture length. When the aperture length is larger than the least upper limit value of the length of the pin hole and equal to or smaller than the least upper limit value of the length of the perforation, the aperture is decided as the perforation 8. When the aperture is decided as the perforation 8, the time between the detection of the perforation 8 and the detection of the next perforation, or feeding amount is detected as the length between the perforations. The step number of the film housing sheet is detected by the length between the perforations.