In a multichannel DC-DC converter that reduces radiation noise to a minimum, a wiring line between the coil conductor of a channel having the smallest load current and the switching IC, among the coil conductors defining a plurality of channels, is the longest connection wiring line, such that a channel having the smallest load current is connected to a wiring line that is most likely to radiate noise and radiation noise is reduced to a minimum. A connection wiring line connected to a coil conductor having the largest load current among the plurality of coil conductors is the shortest connection wiring line. A channel having the largest load current is connected to a wiring line that is least likely to radiate noise and, as a result, radiation noise is further reduced.