PURPOSE: To enable the reception of an attenuated signal by detecting a peak level of a reception signal through the use of a differentiation circuit and setting a threshold level based on the peak level so as to set a proper threshold level.

CONSTITUTION: A signal resulting from eliminating noise from an input signal SIN is inputted to comparators 3, 6 via a differentiation circuit 2. Outputs of the comparators 3, 6 are respectively used for timing signals for sample-and-hold circuits 4, 6. As the result, the sample-and-hold circuits 4, 7 hold a peak level of an input waveform. A threshold level is set based on the held peak level to attain stable reception at all times even when the level is fluctuated.