PURPOSE: To enable exact judgement of whether the mounting condition of a structural body is defective or nondefective by detecting the rotation of the rotating body itself in said structural body.

CONSTITUTION: A detecting means 14 is placed below a magazine 13 which is held mounted in a normal position. The means 14 consists of a pair of photosensors S1, S2 which can detect the presence or absence of a detecting mark 15 provided to one side part of a photosensitive belt 9. When the photosensitive body 9 continues to move, the mark 15 passes periodically through the means 14 and therefore the detection signal is periodically issued and is fed via an amplifier to a CPU. When the mark detection signal is detected by the CPU, the presence of the belt 9 in the prescribed position and the specified movement thereof are known and the housing of the magazine 13 in the prescribed position is indicated. When the magazine 13 is housed in the position deviated from the prescribed position, the mark 15 does not enter the detecting region of the sensor 14 even if the body 9 moves and therefore the mark detection signal is not generated.