A DC power source apparatus has a DC power source for supplying a DC voltage, a transformer, and a switching element connected to a primary winding of the transformer. The switching element carries out ON/OFF operations to convert the DC voltage from the DC power source into high-frequency power, which is transferred to a secondary winding of the transformer and is converted into a DC output voltage. The primary winding of the transformer consists of a first primary winding made of a plurality of winding layers and a second primary winding made of a plurality of winding layers. The first and second primary windings are connected in parallel. The first primary winding is arranged on an inner side of the secondary winding, and the second primary winding is arranged on an outer side of the secondary winding. A terminal of a winding layer farthest from the secondary winding among the winding layers of each of the first and second primary windings is connected to the switching element.