PURPOSE: To prevent chattering having no dull output waveform, by releasing the reset mode of a shift register with a contact signal and at the same time feeding a shift input.

CONSTITUTION: When a contact S is closed, the flip-flops FF1∼FF4 are reset to set an output OUT at a low level. In this case the FF1∼FF4 are reset before the inversion of the FF4 althrough the contact S has chattering. Thus no chattering is caused to the output. When the contact S is opened, the reset mode is released for the FF1∼FF4. Then the FF4 is inverted at the 4th clock CK to set the output at a high level after the chattering is eliminated for the contact S.