If receiving circuit (2) is in a high-sensitivity mode, receiving circuit (2) is switched to a low-sensitivity mode when the BER according to BER measurement circuit (6) becomes more favorable than a release threshold. If receiving circuit (2) is in the low-sensitivity mode, receiving circuit (2) is switched to the high-sensitivity mode when the BER according to BER measurement circuit (6) becomesmore adverse than a start-up threshold. If switching between the high-sensitivity and low-sensitivity modes occurs, control is exercised so that switching from the high-sensitivity mode to the low isresistant to occurring, thereby suppressing the occurrence of a reception error caused by switching between the receiving modes.