An electric power source device has a transformer, primary side semiconductor components, secondary side semiconductor components, a choke coil, a base plate and first circuit substrate and one or more second circuit substrates. The transformer has a primary side coil and a secondary side coil. The primary side semiconductor components form a primary side circuit connected to the primary side coil. The secondary side semiconductor components form a secondary side circuit connected to the secondary side coil. The choke coil has a smoothing circuit for smoothing an output voltage. The transformer, the primary side semiconductor components, the secondary side semiconductor components and the choke coil are formed on the base plate. The first circuit substrate is arranged parallel to the base plate. The second circuit substrate is arranged parallel to a normal line of the first circuit substrate.