To provide a bipolar/unipolar conversion circuit capable of attaining a stable operation, by enabling bias with a margin to be set with respect to the amplitude of an input signal at all times.

An RZ conversion circuit 8 compares a bipolar signal with a bias voltage from a bias-generating circuit 7, outputs "1", when the amplitude of a positive pulse of the bipolar signal is greater than the bias voltage, and outputs "0", when the amplitude of the positive pulse of the bipolar signal is smaller than the bias voltage. An upper limit detection circuit 9 outputs a bias value stored, when the bias value from the bias generating circuit 7 returns to a maximum value. A lower limit detection circuit 10 outputs a bias value stored, when the bias value from the bias-generating circuit 7 returns to the maximum value. A bias arithmetic circuit 11 calculates an average between the bias voltage from the upper limit detection circuit 9 and the bias voltage from the lower limit detection circuit 10, and outputs the average to RZ conversion circuits 1, 2.