PURPOSE: To reduce the capacities of an emergency generator and its driving prime mover by switching the charging current command value to a small value, being linked with power change-over working, and by using most of power generated by the emergency generator for a load.

CONSTITUTION: When an emergency generator 2 is operated due to the service interruption of a commercial power source 1 and its voltage is established, then by an operation circuit as not shown in figures, a power change-over switch 4 is switched to the side of the emergency generator 2. Interlocked with this, a setting changing switch 30 is closed, and then, a current command value set by a current setting device 25 is changed to a small value. As a result, a limit is applied to a voltage regulator 23 so that charging current to a battery 10 cannot exceed this changed current command value, and so DC intermediate- circuit voltage is suppressed. As a result, most of the power generated by the emergency generator 2 can be applied to a load 9 through an inverter 6.