It is possible to provide a method for judging that a failure has occurred without detecting it as an intruding object when a reception signal fluctuates in a receiver by a failure generated in a leaking cable and a device associated with it and calculating a position where the failure has occurred. A delay time from the transmission time of a transmission signal to the reception time of the reception signal is correlated to a transmission path distance based on the reception signal propagation position in radio radiation means (201) and radio reception means (301). Among range bins correlating the reception signal to the distance from the feeding end of the radio radiation means (201) and the radio reception means (301), the range bin corresponding to the far end is checked whether an amplitude reduction width of the reception signal compared to the transmission signal exceeds a predetermined ratio. If yes, it is judged that the radio radiation means (201) or the radio reception means (301) has failed.
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