A voltage measuring device includes a sample-and-hold circuit which alternates between (a) a sample mode in which a signal input from a cell to be measured is sampled so that a voltage across a first capacitor becomes equal to a voltage across the cell to be measured, and a voltage across a second capacitor having its one end connected to one end of the first capacitor becomes equal to zero volts and (b) a hold mode in which the connection between the first and second capacitors is opened, the first capacitor establishes a connection between an inverting input terminal and a non-inverting output terminal of an operational amplifier, and the second capacitor establishes a connection between a non-inverting input terminal and an inverting output terminal of the operational amplifier, and an analog-to-digital converter which converts a signal output from the operational amplifier into a digital signal.