To surely detect whether or not the leading edge of a film in a cartridge is set at the position capable of being taken out by a simple device without causing erroneous detection due to noise by detecting the distortion of a flexible member inserted in the cartridge.

A flexible thin plate 1 is provided with a strain gauge 2 as a distortion detection means and an output part 3 for outputting a result detected by the gauge 2. The leading edge part of the plate 1 is preferable as the place of the gauge 2. Besides, this detection device is provided with a flexible thin plate for pressing 7A above the plate 1 and a flexible thin plate housing case 8 in which the plates 1 and 7A can be inserted and pulled out in a front-and-back direction. Then, the plate 1 provided with the gauge 2 and the plate 7A are inserted outside the film 4 in the cartridge 5 so as to detect the leading edge of the film.