In an electric power supply device for supplying electric power to an electronic apparatus, while a first power source is in a state of being connectable to a first power supply path which is arranged to supply electric power directly to the electronic apparatus when the first power supply path is connected to the first power source, a second power source becomes unconnectable to a second power supply source which is arranged to supply electric power directly to the electronic apparatus when the second power supply path is connected to the second power source. While the second power source is in a state of being connectable to the second power supply path, the first power source becomes unconnectable to the first power supply path. With its simple and low-cost arrangement, the device not only effectively prevents duplex power feeding from different kinds of power sources and a voltage drop but also excels in operability and safety.