PURPOSE: To easily detect interruption of a reception signal by detecting a signal interruption due to increase in noise caused by the interrupt of the reception signal.

CONSTITUTION: A reception amplifier reaches a maximum gain at interruption of a reception signal and lots of frequency components are included in a noise signal wave. In this case, only a clock frequency component of a transmission line goes to a 0 level and other frequency component signal remains, then the signal is outputted from a delay/adder means 110 as a signal amplitude. An output of the delay/adder means 110 is fed to a signal interruption detection means 130 to detect signal interruption as a difference between a 0 level at normal state and a noise component at interruption of the reception signal. Thus, the interruption of the reception signal is easily detected.