A DC-DC converter as an electric power source device has a magnetic component, a semiconductor component, a circuit substrate and a thermal conductive component. The magnetic component has a thermal energy discharge surface. The semiconductor component is electrically connected to the magnetic component. A controller arranged on the circuit substrate is electrically connected to the semiconductor component to control behavior of the semiconductor component. The thermal conductive component is arranged in a thickness direction of the circuit substrate to be overlapped with the magnetic component. The thermal conductive component is arranged, without being electrically connected with the semiconductor component and the circuit substrate, to face the thermal energy discharge surface of the magnetic component to allow thermal energy generated in the magnetic component to be conducted with the thermal conductive component.