PURPOSE: To detect breakage of a belt in its early stage by optically picking up an image of a belt of a belt conveyer, forming the picked up image on a photocell to detect a change of the magnitude of an electric signal sent out from the photocell.

CONSTITUTION: An image pickup device 6 is adapted to approach a hopper 2 and pickup an image of the rear of a belt 3, and the pickup image is formed on a photocell 13. An image pickup device 7 having a lens 7a is disposed apart from the hopper 2 to pickup an image of the rear of the belt 3 and form the image on a photocell 14. A differential amplifier 8a is adapted to compare an electric signal sent from a photoelectric element of the photocell 14 with an electric signal sent from a photoelectric element of the photocell 14 corresponding to the above photoelectric element, and send out a comparison result to a summing amplifier 8b. In this case, if a fissure is caused in the belt, the magnitude of the electric signal output from the photocell 13 is different from that of the electrical signal output from the photocell 14. Thus, a fissure can be detected.