Abstract: A print detector for use in a sorter capable of detecting photoprints on each of a plurality of print carriers so as to prevent photoprints from being erroneously left on any print carrier. Print carriers are mounted at equal intervals on an endless belt adapted to be driven in one direction. Each print carrier has an optical path block mounted on its underside, i.e. the side that faces downward while the carrier is being fed downward. Each print is also provided with a transparent portion along an optical axis of light reflected obliquely upward by a reflecting end face provided at one end of the optical path block. A light emitter emits light to a light incoming end face of the optical path block. A light interceptor is provided on the optical axis of the light reflected by the reflecting end face, and an alarm device is adapted to be activated if the light interceptor fails to receive light emitted by the light emitter.