The invention includes: charging/discharging devices including storage batteries and having a grid-connected function to perform charging/discharging operations on an electric power system and an islanded-operation function to supply the electric power system with a constant-voltage constant-frequency electric power; a monitor detects an electric power state of the charging/discharging device and to transmit detection output to the charging/discharging device; and a switching unit connects and disconnects commercial power source to and from electric power system. When a commercial power source is normal, the switching unit connects the commercial power source to the electric power system and charging/discharging devices establish a grid connection. When commercial power source fails, switching unit disconnects commercial power source from electric power system, charging/discharging device performs an islanded operation, and charging/discharging device performs a charging/discharging operation according to a detection output of monitor and performs control so that islanded operation electric power of charging/discharging device becomes constant.