A DC-DC converter includes a voltage input portion that receives a DC voltage, a voltage output portion to which a load is connected, a transformer that includes at least a primary winding and a secondary winding, a main switch element between the voltage input portion and the primary winding of the transformer, a rectifier circuit between the voltage output portion and the secondary winding of the transformer, and a reset voltage suppressing circuit that detects a magnitude of a reset voltage generated when excitation energy accumulated in the transformer is reset and reduces a voltage which is applied to the main switch element when the magnitude of the reset voltage exceeds, for example, a predetermined threshold value.