A receiving module selects frequencies to be used for data request signals from all channels if there is no channel prohibited from being used, and if there are channels prohibited from being used, the receiving module selects frequencies to be used for data request signals from channels other than the channels prohibited from being used. In the case failed in acceptance of measured data from a sensor module, the receiving module sets communication channels failed in acceptance of measured data predetermined times to prohibited channels from being used. In this manner, when the receiving module transmits the data request signals to the sensor module, frequencies of channels having jamming radio waves and the like could not be selected by the receiving module, thereby enabling the receiving module to select the frequencies of high probability in communication reception.