Abstract: In an uninterruptible power system with a battery pack (100) connected replaceably and conductively to a power control device (50) by a connector (104), the battery pack (100) and the power control device (50) are encased in a main box (10) having a battery pack load/unload opening (12). The battery pack load/unload opening (12) is adapted to be opened/closed by a protective cover (14). The opening of the protective cover (14) is detected by a protective cover opening detection switch (22) for detecting the opening of the protective cover (14). When opening of the protective cover (14) is detected by the protective cover opening detection switch (22) while the battery pack (100) is being charged or the backup power is being supplied by the battery pack (100) during a power failure, an internal switching circuit (60) of the power control device (50) is forcibly turned to a substantially open state or an alarm is output by an alarm buzzer (70).