A converter whose output voltage can be changed by a three-level PWM control smoothly and continuously from zero to the maximum. The converter comprises multi-pulse generating means for generating PWM control signals corresponding to bipolar modulation mode, unipolar modulation mode, and overmodulation mode; one-pulse generating means for generating PWM control siganls corresponding to one-pulse mode; and means for controlling the transition between the modulation modes. The transition between the unipolar modulation mode and one-pulse mode is so made to pass through the overmodulation mode. <IMAGE>
