In a switching power supply device, a partition portion that includes a slit divides a winding portion of a bobbin. A primary winding of a transformer is wound to a height h1 in a first section, and a secondary winding is wound to a height h2 in a second section. A low side drive winding and a high side drive winding are wound around the primary winding to a height h3 with the high side drive winding being located toward the secondary winding.