PURPOSE: To eliminate the limit of an installing place regardless of environmental conditions by calculating the position of a heavy machinery from a light receiving direction detected by each light receiving device, comparing the position with a warning area set in advance and issuing an alarm when the heavy machinery is positioned in this area.

CONSTITUTION: A light emitting device 1 to emit light in all the directions is fitted to the top of the boom of a crane 10. On the other hand, light receiving devices 2a and 2b to receive the light from the light emitting device 1 are fitted to two or more positions (such as the legs of steel tower, for example,) sandwiching a desired spatial monitoring area to be monitored. These light receiving devices 2a and 2b measure the direction (horizontal and vertical angles) of the light coming from the light emitting device 1 and calculate the position of the light emitting device 1, namely, of the crame from the measured direction of the light while using the method of a triangular measurement. Thus, the light receiving direction of the light from the light emitting device 1 is detected, the position of the light emitting device 1 is detected and compared with the spatial warning area set in advance from the light receiving direction detected by the respective light receiving devices 2a and 2b, and the alarm is issued when the crane is positioned in the warning area.