To provide a DC-DC converter that prevents variation in output power due to variation in the number of input conversion circuits executing switching operations.

PWM control is executed in a DC-DC converter 100, composed by connecting a plurality of input conversion circuits in parallel, in a discontinuous current mode. When target output power is larger than the sum of maximum output power of the input conversion circuits executing switching operations, any one of switching operations of each input conversion circuit not executing the switching operations up until then is started so as to adjust its duty ratio while holding all of the input conversion circuits executing the switching operations up until then at a maximum duty ratio. When the target output power is smaller than the sum of maximum output power of the input conversion circuits executing the switching operations away a plurality of the input conversion circuits, any one of duty ratios of the input conversion circuits executing the switching operations up until then is adjusted while the remaining input conversion circuits are set to have a maximum duty ratio.