The present invention provides a DC/DC converter, a DC/DC converter apparatus, vehicle, fuel cell system and a method for driving DC/DC converter, to resolve minimization of three-phase DC/DC converter. A DC/DC converter (36) comprises a plurality of phase arms (UA, VA, WA), the phase arms (UA, VA, WA) being connected parallel to each other between the cell 24 and the fuel cell 22, each of the phase arms comprises a series-connected circuit of an upper arm switching device (81u, 81v, 81w) and a lower arm switching device (82u, 82v, 82w) and diodes (83u, 83v, 83w, 84u, 84v, 84w) connected inversely. Wherein the phase arms (UA, VA, WA) have respective midpoints connected to each other, and the single reactor (90) is inserted between the connected midpoints. According to the construction, the DC/DC converter (36) may be carried out with conversion of voltage of the phase arm (UA, VA, WA) by using a reactor (90).