The power consumption of a battery for supplying electric power to a cassette having a radiation detector for detecting radiation image information is greatly reduced. When a cassette transceiver of the cassette starts transmitting the radiation image information to a console transceiver of a console, the cassette transceiver changes the gain of a variable-gain amplifier to change a transmission radio-wave intensity, and transmits a test signal at the changed transmission radio-wave intensity. When the console transceiver receives the test signal, the console transceiver transmits a reception acknowledgement signal generated by a reception acknowledgement signal generator. In response to the reception acknowledgement signal, the cassette transceiver sets its own transmission radio-wave intensity to a value at the time the cassette transceiver received the reception acknowledgement signal, and transmits the radiation image information at the set transmission radio-wave intensity.