To prevent a transparent void part in a frame from being erroneously decided as a space between the frames, to correctly detect the frame and to prevent the frame from being erroneously retrieved by binarizing a film density signal and deciding the existence of the frame by using the AND or the OR of the plural binarized signals.

Optical sensors (a)-(e) detecting the density of a microfilm 1 are arranged at equal intervals in the width direction of the film 1. The binarized signals obtained by binarizing the outputs of the sensors (a)-(e) is logic 1 (H level) at the outside (transparent void) of the frame 2 and logic 0 (L level) at the background part of an original in the frame 2. Therefore, when the void part 3 of the frame exists on the scanning loci of the sensors (a)-(e), the outputs of the sensors (a)-(e) become the logic 1. The AND of the whole binarized signals becomes the logic 0 in the frame 2 and becomes the logic 1 between the frames 2 (space between the frames). By integrating the changing frequencies of the AND to 0 from 1, the number of frames can be integrated and the frame can be retrieved.