A battery management circuit with a novel structure and a power storage device including the battery management circuit are provided. The power storage device includes a plurality of battery cells connected in series and a battery management circuit. The battery management circuit includes a voltage monitor circuit having a function of acquiring a voltage value between a pair of electrodes of any one of the battery cells. The voltage monitor circuit includes a multiplexer and a buffer circuit for outputting a signal for controlling the multiplexer. The multiplexer and the buffer circuit each include an n-channel transistor. The n-channel transistor is a transistor including an oxide semiconductor in a channel formation region. The multiplexer has a function of retaining an output voltage of the battery cell by setting the transistor in an off state.