PURPOSE: To speed up responce by adopting a sample holding technique in case of converting a pulse train into a DC output.

CONSTITUTION: An integrated circuit 6 and a differential shaping circuit 10 are connected in parallel to an output terminal of a pulse train generating circuit 1. A sample holding circuit part 14 consisting of a field effect transistor 11 of which drain is connected to the output terminal of the integrated circuit 6 and a gate is connected to the output terminal of the differential shaping circuit 10, a capacitor 12 connected to the source of the field effect transistor 11 and a buffer circuit 13 formed by using an operational amplifier is connected to a titled converter. The output from the sample holding circuit part 14 is taken out through a low-pass filter 22 and outputted from a DC output terminal 3.