A coil substrate structure which enhances heat dissipation and fully secures a mounting area is provided. A coil substrate structure 100 comprises a first coil substrate 110 having a primary transformer coil part 41; a second coil substrate 120, disposed on the first coil substrate 110, having a secondary transformer coil part 42; and a transformer core 130 for magnetically connecting the transformer coil parts 41, 42 to each other. The coil substrates 110, 120 are disposed on each other while being shifted from each other such that the transformer coil parts 41, 42 overlap each other as seen in the substrate thickness direction. This can increase the heat dissipation area of the coil substrates 110, 120. The transformer coil parts 41, 42 have a width in a transmission direction A narrower than a width in a direction B intersecting the transmission direction A as seen in the substrate thickness direction. This can reduce the multilayer area of the coil substrates 110, 120 in the transmission direction A.