PURPOSE: To cause the low pass filter of a final stage to be sufficient with a simple constitution to remove a high frequency component by using a voltage generating circuit and a sample holding circuit and converting the pulse rate of an input pulse to a voltage.

CONSTITUTION: Since a voltage OE at a time F can be held by going back to the period of F from a time O, the holding of the voltage is dislocated for one period, and for the period of a pulse interval after a second pulse GHFI occurs, the voltage OE is held and then, a pulse rate can be converted to the voltage. Consequently, a voltage generating circuit 1 generates a voltage which comes to be v=c/t in accordance with the fall of an input pulse P, a voltage c/t is sampled for the 'H' level period of the input pulse P by a sample holding circuit 2, the voltage sampled for the 'L' level period is held, the output of the sample holding circuit 2 is inputted to a low pass filter 3 and conversion is realized.