PURPOSE: To decrease the number of control processes and to attain the control of an offset voltage at all times by controlling automatically the offset voltage of an integrator.

CONSTITUTION: A digital signal having a mark factor of 50%, for example, is applied to the integrator 5. The output voltage of the integrator 5 is converted into the digital voltage by a converter 8-3 via a multiplexer 8-1 and applied to an MPU7. The MPU7 reads out the offset voltage corresponding to the output voltage 0 stored in a ROM10 as long as the input voltage of the MPU7 is not equal to 0. The offset voltage is applied to the integrator 5 via a multiplexer 8-2 and a converter 9-2. When the output voltage of the integrator 5 is equal to 0, the control is through with the offset voltage. Then a latch circuit 9-1 self-holds the supplied offset voltage, and another integrator is selected to repeat the above-mentioned procedure. For the integrator 5 with which the control is through, the offset voltage held at the circuit 9-1 is converted into the analog voltage by a converter 9-2 and applied to an amplifier 5-1.