PURPOSE: To provide the PWM modulation circuit not generating a modulated wave asymmetrical in the vertical direction with respect to a modulation wave symmetrical in the vertical direction.

CONSTITUTION: The PWM modulation circuit is provided with a comparator means 1 detecting it that a modulation wave A is higher than a carrier B, a same level detection means 2 detecting whether or not the modulation wave A and the carrier B have a same level, a bit detection means 3 detecting a most significant bit Ba of the carrier, and modulation means 4, 5 applying arithmetic and logical operation to am output signal of the comparator means 1, an output signal of the same level detection means 2, and an output signal of the bit detection means 3 to modulate the modulation wave A so that the modulation wave A and the modulated wave C are symmetrical in the vertical direction.