To obtain a reverse current prevention circuit that prevents discharge from a smoothing capacitor to the utmost when an input end is short-circuited in a DC/DC converter, prevents the breakage of a component when the input end is reversely connected, and operates with a low loss in a normal state.

This invention relates to a reverse current prevention circuit of a DC/DC converter. The reverse current prevention circuit comprises: a switching means that is arranged on an input line, and switches the conductivity of the input line; an input voltage detection means that sets a threshold for switching the switching means, and detects whether or not the input voltage has reached the threshold; and a switching control means that controls so that the switching means is closed when the input voltage has reached the threshold, and the switching means is opened when the input voltage has not reached the threshold.