PURPOSE: To realize a low-pass filter by making use of a few parts, by inserting a bypass capacitor between an output end of an optional stage in an operational amplifier and an electric power supply end.

CONSTITUTION: In case of input voltage vi=0, half of an output current I1 of a constant-current source CS1 flows to each FETQ1, Q2. Voltage of an output terminal 25b is "0". When the voltage vi rises, the FETQ1 and the FETQ2 become off and on respectively, and the current I1 flows to the FETs Q2, Q4. As a result, a mirror current I1 flows to the FETQ3, and a bypass capacitor 26 is discharged. Therefore, gate voltage of the FETQ5 drops, and output voltage V0 rises. When the voltage vi returns to "0", the FETQ1 becomes on. The bypass capacitor 26 is charged. When frequency of the voltage vi becomes high, charge and discharge are not executed sufficiently, and a high-cut characteristic is offered. This low-pass filter is suitable for those devices which do not require a sharp characteristic, such as a process converting device.