A planar array coil includes a multilayer substrate at which a plurality of coils are formed and arranged on a plane of the multilayer substrate. First ends of the plurality of coils are connected to switching circuit units of a power conversion circuit, and second ends of the plurality of coils are connected to a common output part. The coils include two coil parts in which currents flow in opposite rotation directions, and one coil part of one coil is adjacent to one coil part of another coil. The plurality of coils are connected such that operation of switching circuits causes currents in regions of adjacent coil parts that extend in parallel to each other to flow in the same direction. Accordingly, adverse effects by unwanted coupling between adjacent coils are reduced and a switching power supply device that includes the planar array coil performs stable power conversion operations.