PURPOSE: To stably perform a signal disconnection detection by performing a phase modulation for the output of an equalization amplifier part by the modulation signal of a fixed frequency.

CONSTITUTION: When an input signal exists in an equalization amplifier part 100, a phase modulation is performed for the output by the modulation signal of a fixed frequency f1. As a result, a signal for which a pulse width modulation is performed, for instance, can be obtained from a phase modulation part 140. By passing this modulated signal through a filter part 160, a signal of a fixed amplitude including frequency f1 component is outputted, the presence of the signal is decided in a signal disconnection detection part 180. When the output noise of the amplifier part 100 exists at the time of no input, a phase modulation signal can not be obtained even if a modulation is performed by the modulation signal of the f1 in a modulation part 140. Therefore, the f1 component is not included in the output of the filter part 160, the output amplitude is small and a signal disconnection can be stably detected in a detection part 180.