PURPOSE: To detect a cargo accurately, by installing an injection nozzle blasting liquid to a conveying area of specimen and pressure receiving members arranged against the nozzle with the cargo set in between and displacing backwards corresponding to existence and absence of the pressure of the liquid injected from the nozzle and a switching member acting by the displacement of the pressure receiving member.

CONSTITUTION: High-pressure water is injected upward constantly from a nozzle located at the lower part of a cargo conveying area and the jet strikes the receiving part 14 to swing it upward and brings it roughly horizontal. By this a swing shaft 12 is brought to a swinging motion, a reflector plate 19a fixed on its end deviates from the light projected area of a photoelectric switch 19c and the switch C is placed in OFF condition. When a specimen 30 is conveyed to reach the projecting area of the nozzle 16, the high pressure water does not reach the receiving part 14. By this arrangement, the receiving part 14 swings downward by its own weight and the reflector plate 19a is swung up to the inside of the projected light area and the photoelectric switch 19c is placed in the ON condition.