A DC-DC converter includes a regeneration preventing circuit to bring a synchronous rectifying switching element to an OFF-state when an output of an error amplifier is lower than a lower-limit potential of a triangular-wave signal, that is, when the output of the error amplifier deviates from a voltage range of the triangular-wave signal in a side (direction), which would cause a smaller ON duty ratio of a main switching element. This configuration prevents a rise of voltage in an input side due to a regenerative operation at startup when a pre-bias voltage exists. Also, the configuration prevents fluctuations of an output voltage at a sudden change of a load current and a drop of the output voltage at transition from a soft start state to a normal operation state when the load current is small.