A battery pack is provided which includes: a battery set having a plurality of secondary-battery cells connected in series and in parallel; and a battery-abnormality detection circuit. The battery-abnormality detection circuit controls a battery-set circuit separation switch, so as to detect an abnormality in each secondary-battery cell based on a change in the voltage of each cell when parallel-connection separation switches are turned off, and if detecting an abnormal secondary-battery cell, so as to separate the series module including this cell from a battery-set body.