a digital - signal forming circuit demodulates a time - divisionally received signal , and shapes the demodulated signal by comparing the demodulated signal with a reference voltage . when a non - receiving time slot has a small interference signal , the digital - signal forming circuit uses , as the reference voltage , a direct - current voltage obtained by averaging the demodulated signal . when the non - receiving time slot has a large interference signal , the digital - signal forming circuit uses a predetermined direct - current voltage as the reference voltage .