PURPOSE: To obtain an apparatus with ripples suppressed to a small quantity by making the gain of a feedforward circuit proportional to one over the square of the value of the DC component of an input voltage.

CONSTITUTION: A gain compensating circuit 18 always controls the gain of a feedforward circuit 10 to optimize the gain. The gain compensating circuit 18 divides the DC component of an input voltage by voltage divider resistors 19, 20 and a capacitor 21, and the value of the divided voltage is squared by a multiplier 22. Further, the outputs of the feedforward circuit 10 and the multiplier 22 are inputted to a divider 23 to change the gain of the feedforward circuit 10 in proportion to one over the squared of the value of the DC component of the input voltage. Therefore, even if the DC component of the input voltage changes, the gain of the feedforward circuit 10 can be maintained always at the optimum value; and when the output voltage ripple suppression effect of the feedforward circuit 10 is taken out, it is possible to raise the cut-off frequency of an input filter and to miniaturize an apparatus.