A bit-rate automatic control circuit for feedback controlling the gain and bandwidth of a preamplifier in dependence on the bit-rate of a signal is disclosed. The circuit includes a bit-rate discrimination circuit configured to discriminate the bit rate of the signal, and an alarm circuit configured to determine whether the value of received power of the signal is less than the minimum acceptable value of the preamplifier or not. If said alarm circuit determines that the value of received power of the signal is less than the minimum acceptable value of the preamplifier, the bit-rate automatic control circuit determines, irrespective of the discrimination of said bit-rate discrimination circuit, that the bit-rate of the signal is low, and feedback controls the preamplifier based on the determination. According to the above arrangements, if a determination is made that the reception level of the input signal is lower than the minimum acceptable value of received power, the preamplifier can be switched from a state for receiving a high bit-rate signal to a state for receiving a low bit-rate signal.