A DC-DC converter transforms a DC input voltage to generate a DC output voltage by complementary switching control of a main switching transistor and a synchronous rectifying transistor. The DC-DC converter includes a soft-start circuit configured to generate a soft-start voltage rising from an initial voltage at start-up of the DC-DC converter; and a control circuit configured to control switching of the main switching transistor and the synchronous rectifying transistor based on the soft-start voltage to perform soft start of the DC-DC converter. The control circuit brings both of the main switching transistor and the synchronous rectifying transistor to an off state while the soft-start voltage is lower than the DC output voltage.