When a cassette transceiver starts to transmit radiation image information to a console transceiver, the cassette transceiver transmits a test radio wave, and the console transceiver calculates a minimum transmission radio-field intensity of said cassette transceiver which is required to receive the radiation image information from a received intensity of said test radio wave, and transmits a command radio wave representing the calculated minimum transmission radio-field intensity as a command to said cassette transceiver. The cassette transceiver is thus capable of transmitting the radiation image information at the commanded minimum transmission radio-field intensity. The consumption of electric power for transmitting the radiation image information from the cassette transceiver to the console transceiver is thus minimized.