A power supply device for an electromotive railcar is adapted to receive external current from an overhead electric line via a pantograph and to feed driving current of lower voltage than the external current to electric loads in the railcar through first and second current output lines. The power supply device is provided with a static inverter couplable to the overhead electric line through first and second input lines and capable of producing the driving current. A battery is connected to the static inverter in parallel with the loads for selectively charging the driving current and discharging the charged electric current to the loads. An anti-discharge switch is provided on the first current output line for selectively connecting and disconnecting the loads to and from the battery. A discharge controller coacts with the anti-discharge switch for allowing the anti-discharge switch to be closed when no external current is fed to the static inverter and for causing the anti-discharge switch to become open when the static inverter receives the external current from the overhead electric line. In addition, the power supply device includes an emergency switch manually operable and in cooperation with the discharge controller for causing the anti-discharge switch to be closed so that the electic current stored in the battery is fed to the loads at the time the static inverter delivers no driving current to the loads.