Abstract: A transformer includes a core group and a winding group. Each of a plurality of winding layers penetrates through a plurality of closed magnetic circuits formed by a plurality of cores. When viewed from the top, the cores are disposed with spacing along conductor wires through which current of the winding layers flows, and the winding layers are disposed side by side from the inside to the outside with spacing along a direction intersecting the direction of current. With this configuration, a coil device with good heat dissipation performance, compact size, high efficiency, and low costs can be provided.