A protection device for an assembled battery includes a sampling capacitor, first switches and second switches to sample the voltages of the battery cells and hold the voltages in the capacitor, a detection unit which detects the voltages of the battery cells based on the voltage sampled by any one of the first switches and the second switches and held on the capacitor and output the detected value, a computing unit which computes an average value of the detected values, a comparator which compares the detected value with the average value so as to obtain a result of comparison indicating a relation in size between the both, and a controller which controls the first switches and the second switches, for the second switches to sample if the detected value obtained by sampling by the first switches is higher than the average value according to the comparison result.