Power storage device and method of measuring voltage of storage battery

A power storage device has a plurality of series-connected storage battery units (101), battery circuits (102) associated with the storage battery units to control or monitor the storage battery units, respectively; a main circuit (104) of a potential level different from that of the battery circuits; and a potential level changing circuits (103) connecting the battery circuit to the main circuit. The power storage unit is small in construction and operates at a low power consumption in a high control accuracy.