PURPOSE: To detect the approach status to a master station by a constitution of simple equipment by using such a relation that the number of pulse signals to be received by a slave station changes according to the distance from the master station.

CONSTITUTION: The signals received by a reception antenna A2 are detected by a logarithm detector 25, the signals are added to a modulation signal detector 26 and a level detector 27 and a signal level is detected. In this case, when a slave station approaches a master station, all the pulses are detected, and as the slave station is for from the master station, the number of pulses to be detected becomes fewer. Therefore, which pulse among five pulses, for instance, has been detected is decided by the frequency of the modulation signal to be detected in a modulation signal detector 26, the decision result is delivered together with a reception level to a reception control computer 28 and the change in the distance of a reception level according to a preliminarily fixed transmission output level is controlled with a computer. As a result, when both stations approach each other and the state is judged to be danger, an alarm device 29 is operated.