To provide a line quality monitor that attains a high-speed operation and detects an error with high accuracy.

The line quality monitor is provided with a 1st comparator 15 that has a feedback circuit that compares a reception signal interrupting a DC component with a signal short-circuited in terms of AC to compensate a DC component of an output signal, a 2nd comparator 16 that has a feedback circuit that compares the received signal interrupting the DC component with a low frequency signal interrupting the DC component to compensate the DC component of the output signal, an amplitude detector 13 that detects the amplitude of the received signal, an amplitude controller 14 that controls the amplitude of the low frequency signal on the basis of the amplitude of the detected reception signal, an exclusive OR circuit 23 that receives outputs of the 1st and 2nd comparators 15, 16, an identification unit 24 that identifies an output of the exclusive OR circuit 23 at an identification phase of a clock signal extracted from the received signal and an error rate calculation section 25 that calculates a code error rate from the output of the unit 24.