A DC-DC converter has a primary-side circuit connected to a DC power supply and a converter for converting voltage supplied by the DC power supply into an AC signal; a secondary-side circuit for rectifying and smoothing the AC signal transmitted from the primary-side circuit to supply power to a load; and an auxiliary rectifying and smoothing circuit for rectifying and smoothing the AC signal transmitted from the primary-side circuit to supply sub-power supply voltage to the converter of the primary-side circuit. The primary-side circuit further has a monitoring circuit for monitoring the sub-power supply voltage supplied from the auxiliary rectifying and smoothing circuit to the converter and a stop circuit for latching a conversion operation of the converter into an OFF state.