A semiconductor device for monitoring batteries or cells connected in series has a selector switch that selects one of the batteries or cells and outputs voltages obtained from its positive and negative terminals. A pair of buffer amplifiers receives these voltages at high-impedance input terminals and output corresponding voltages to a level shifter. The level shifter generates an output voltage equal to the difference between the outputs of the buffer amplifiers. By preventing current flow between the selector switch and the level shifter, the buffer amplifiers reduce the output droop that occurs at the beginning of a voltage measurement, even if the semiconductor device is connected to the batteries or cells through a low-pass filter circuit with a comparatively large time constant. Measurement time is shortened accordingly.