An electric power source device has a transformer, a primary-side semiconductor module, a secondary-side semiconductor module, a secondary-side electrical component, a base plate and a circuit substrate on which substrate-side electrical components are mounted. The primary-side semiconductor module has a larger exterior size than the secondary-side electrical component. The primary-side semiconductor module and the secondary-side electrical component form a stacked section. In the stacked section, the secondary-side electrical component is stacked, in a vertical direction, i.e. a direction of a normal line of a mounting surface of the base plate, on the primary-side semiconductor module. The primary-side semiconductor module is directly mounted on the mounting surface. At least a part of the substrate-side electrical components is arranged inside of the primary-side semiconductor module in a horizontal direction, and inside of a second surface of the stacked section toward the mounting surface along the normal line.