PURPOSE: To improve high speed response and to reduce the pulsating component of a voltage output, by converting an input frequency into a pulse duty signal proportional to the input frequency, and integrating the converting signal by the time corresponding to the pulse width on the basis of the reference voltage of an integrator.

CONSTITUTION: A frequency pulse duty signal converter 11 to convert frequency into a pulse duty signal is connected to an input termial F. The output side of a reference voltage source 12 is branched and connected to analog switches 13, 14 respectively, and analog switches 15, 16 are connected between the output side of each switch 13 and the earth respectively. The negative input terminal of the 1st integrator 19 is connected to the output side of the switch 13 through a resistor 18, and the negative input terminal of the 2nd integrator 19 is connected to the output side of the switch through a resistor 20. The outputs of the integrators are supplied to an adder 28 through resistors 26, 27 respectively.