A control circuit for a DC-DC converter that prevents a deficiency from occurring due to sudden interruption in power supply. The control circuit activates and inactivates a main switching output transistor and a synchronous rectification output transistor to convert the input voltage to an output voltage. The output voltage is smoothed by a smoothing capacitor. When the input voltage decreases, the control circuit activates the synchronous rectification output transistor to readily discharge the smoothing capacitor via the synchronous rectification output transistor.