PURPOSE: To easily increase a specified constant with a resistor and a capacitor having small values by connecting a switching means for ON/OFF in a fixed cycle to the input side of a low-pass filter circuit to pass a low frequency.

CONSTITUTION: A signal (a) is inputted from an input terminal 11, and the waveform is converted to a pulse-shaped signal (b) by ON/OFF in the fixed cycle by a switch means 13. The pulse-shaped signal (b) is inputted to a low-pass filter 16 and a low-frequency component is taken out from an output terminal 17. The switch means 13 controls the fixed cycle, for example, ON time as TON and OFF time as TOFF according to a switch control signal 12. A duty ratio K is turned to be K=TON/(TON+TOFF) and the value of resistance 14 is apparently made 1/K times. For example, in the case of TON=1μs and TOFF=999μs, the condition of 1/K=1000 is established, and a small ceramic type is usable for the capacity of a capacitor 15 as well. Thus, a filter having satisfactory temperature characteristic without the danger of leaking liquid can be easily produced.