To provide a power source system excellent in reliability, capable of utilizing an alkaline storage battery for a long service life as a main power source in the real usage environment of an HEV.

The power source system is equipped with: a main source comprising an alkaline storage battery; a charging/discharging circuit controlling the charge and discharge of the main source; a voltage sensor detecting the voltage of the main source; an SOC detector sequentially checking a charging level from the voltage detected by the voltage sensor; and a controller recognizing the charging level checked by the SOC detector as a management-required condition when the charging level is not less than A1 and its variation is not more than A2, and setting the level below A1 by discharging the main source using the charging/discharging circuit when the management-required condition continues longer than a predetermined time T1.