PURPOSE: To obtain an addition excitation device having high reliability and safety of an electric rolling stock without stopping the operation of the electric rolling stock even when a power supply for the addition and excitation of the electric rolling stock gets trouble by installing an exclusive power unit for the main-circuit control section of the electric rolling stock.

CONSTITUTION: When an auxiliary power unit 11 for addition and excitation gets trouble, an auxiliary power-unit 11 interlocking relay coil 11c and the relay coil 2c of a main contactor 2 are demagnetized, and the main contactor 2 is released. The relay coil 9c of a weak field contactor 9 is demagnetized, the weak field contactor 9 is released, and the control of addition and excitation is stopped. Since the closing switch 9s of the weak field contactor 9 is not closed by the protective operation, the voltage of a battery 12 excites the relay coil 2c of the main contactor 2, the main contactor 2 is closed again, the currents of a main circuit are made to flow in the path of the armature 3 of a DC series motor, a field winding 4, a bypass diode 7 and a main resistor 6, and the DC series motor is full-field controlled, thus restarting the auxiliary power unit 11.