To prevent the generation of the deviation of a threshold level, and to identify a non-signal by outputting a level held higher than a non-signal level only by a forced offset value at the time of inputting no signal, and outputting the voltage-division value of signal amplitude as a threshold level when the input signal level is more than a prescribed value.

A peak detecting circuit 10 with a dead zone outputs a level almost equal to a non-signal level at the time of inputting no signal, and when the level of the input signal is more than a prescribed value, the circuit 10 outputs a 1 side output level lower only by level shift amounts equal to VOF×[(R1+R0)/R0]. In this case, the VOF indicates a forced offset value, and the R0 and R1 indicate resistance values. A voltage dividing circuit operate voltage division at R1:R0 to 1 side and 0 side output levels, and a level shift circuit 12 outputs a level held higher than the non-signal level only by the forced offset value VOF at the time of inputting no signal, and outputs the voltage-division value at the R1:R0 of signal amplitude as a threshold level when the level of the input signal is more than a prescribed value.