A reception sensitivity is measured at a high precision. An access terminal function portion 126 includes the transmitter part 214 and receiver part 215 of a communication terminal in a radio communication system. A path switch part 132 effects switching so as to connect the input ends of receivers 134, etc. to antennas 116, etc., or to terminate them. Besides, the path switch part 132 switches the paths of a signal from a transmitter 133 and signals toward the receivers 134, etc. An access point controller 115 adjusts a packet error rate into a predetermined range, and it obtains the reception sensitivity on the basis of the transmission power of the access terminal function portion 126 after the adjustment. Alternatively, the access point controller 115 diagnoses the failures of the receiver and the signal path, depending upon whether or not the reception power value of the receiver 134 or the like in the case of connecting the receiver and the transmitter part 214 of the access terminal function portion, and the reception power value of the access terminal function portion in the case of connecting the transmitter 133 and the receiver part 215 of the access terminal function portion lie within predetermined ranges, respectively.