To set optimum bias values even when the amplitude level of an input signal varies.

A bipolar/unipolar conversion circuit 1 includes: RZ conversion circuits 2 and 3 for comparing an input signal I with respective positive and negative bias values V(+) and V(-) and converting the input signal to RZ signals in accordance with the comparison results; an OR circuit 4 for computing the logical sum of the RZ signals; an NRZ conversion circuit 7 for converting a bipolar RZ signal A output from the OR circuit 4 to an NRZ signal; an input interruption detection circuit 10 for detecting an input state of the input signal in accordance with the bipolar RZ signal; a bias setting circuit 12 for setting the positive and negative bias values in accordance with the detection result of the input interruption detection circuit 10; and a code rule error detection circuit 11 for detecting whether or not each RZ signal conforms to a predetermined coding rule. The bias setting circuit 12 sets the positive and negative bias values when each RZ signal is detected not conforming to the coding rule.