PURPOSE: To improve the battery saving ratio, by reducing the duration of the electrode pulse which is periodically supplied and detecting the preamble signal while securing the synchronism for each bit.

CONSTITUTION: The power supply is applied to a receiving part 2 for a prescribed time x1' after closing a power supply switch 5. Thus the part 2 demodulates the receibed signal and transmits it to a control part 8. The part 8 has a monitor to discriminate the preamble signal based on the demodulated signal. If no preamble signal is detected within the time x1', the part 8 controls the switch 5 to cut the application of power supply to the part 2 for a fixed time (y') by a battery saving operation. When the preamble signal is detected for the next time x2', the power supply is secured for a certain time (α+β). Then the acquisition of synchronism is carried out for the first time α to secure the synchronism between the demodulated signal given from the part 2 and the internal clock. In the next time β, the confirmation is given whether the received signal is a preamble signal or not.