PURPOSE: To detect a fault even to a transmitter/receiver in operation by outputting respectively a short time mean value of an instantaneous value of each reception electric field strength from plural radio reception sections and comparing each mean value in the same time to conduct easily the test.

CONSTITUTION: Plural radio reception sections 1 output a demodulated output 2 and an electric field strength signal 3 representing an instantaneous value of a reception electric field strength. A comparator circuit 4 compares the instantaneous values 3 of each reception electric field strength, selects a signal from a reception section 1 having the largest value and outputs a selection signal 5 selecting the reception section 1. A changeover circuit 6 uses the selection signal 5 from the comparison circuit 4 to output a demodulation output 7 of the radio reception section 1. Further, an averaging circuit 8 outputs a short-time mean value of each reception electric field strength 3 respectively, a comparison circuit 9 compares always the outputs from the averaging circuit 8 and when the difference differs remarkably from the mean value of the other reception sections 1, fault information 10 is outputted.