PURPOSE: To detect automatically that a substance to carry is hooked to a hook securely in a crane operating room by detecting the condition the hook is hooked to the substance to carry, transmitting the detecting output as a radio data, and receiving it by a receiving device provided at the crane fixing part.

CONSTITUTION: At the side surface of a hook 1, a reflection type heatproof optical sensor switch 16 which can bear a high temperature of 300°C level is fixed, and protected with a cover 17. When a trunnion 18 projecting from the side wall of a conveying container 19 is hooked to the hook 1 properly, this condition is detected by the optical sensor switch 16, and the output signal to indicate this condition is transmitted by a radio and received by a receiver on the ground. And this condition is displayed on a display panel. When the trunnion 18 is not hooked property to the hook 11, no output is made from the optical sensor switch 16, and such a condition is displayed on the display panel through the transmitter and the receiver on the ground in the same manner.