PURPOSE: To keep an 'L' level period and an 'H' level period unchanged in a detected output independently of an input level.

CONSTITUTION: A PPM signal is inputted to a base of a PNP transistor(TR) 1 and also inputted to a 1st input terminal 53 of a charging circuit 5 and a 1st input terminal 56 of a discharging circuit 6. The charging circuit 5 detects a voltage between the 1st input terminal 53 and a 2nd input terminal 54 and the discharging circuit 6 detects a voltage between the 1st input terminal 54 and a 2nd input terminal 57, and supply and extraction of a current to/from a capacitor 7 are executed properly through an active operation of PNP TRs 10, 12 being components of the charging circuit 5 and the discharging circuit 6 respectively. The rising/trailing speed of a detected output signal of the PPM signal outputted via a buffer 9 is promoted and the detected output signal corresponding to an interrupted waveform of the PPM signal input is accurately reproduced and outputted.