A plurality of microswitches are arranged in a photosensitive material receiving section in a row, in a direction perpendicular to the travel direction of a printing plate. The printing plate is carried while being centered with respect to a center of a transport path. The plurality of microswitches are arranged in an asymmetrical manner with respect to the center of the transport path. Thus the width of a photosensitive material can be precisely calculated with a small number of microswitches. Further, deviation of the photosensitive material from the center of the transport path can be easily detected.