A protection device for an assembled battery includes a sampling unit which individually samples voltages of batteries of the assembled battery to generate sampled voltages, a holding unit which holds the sampled voltages to generate a plurality of holding voltages, a multiplexer which sequentially reads the holding voltages and outputs each voltage to a common output node, a measuring unit configured to measure the individual voltages and the state of charges of the batteries based on a voltage of the node in a measuring period within a constant measuring cycle, and a control unit configured to turn on/off the sample switches at the same time within the measuring period and repeatedly turn on/off at least one sample switch selected from the plurality of sample switches in accordance with the state of charge in a term other than the measuring period.