A voltage conversion device includes a forward direction conversion circuit for converting an input voltage from a DC power source into a desired output voltage by adjusting a duty ratio of semiconductor element switching. The device further includes a control section that controls the duty ratio according to at least a target output voltage and an output voltage so that the output voltage approaches the target output voltage. Based on an operational value of the control section, it is judged whether a failure has occurred. Thus, it is possible to judge whether a failure has occurred without constituting a dual system.