In a battery monitoring device, an amplifier circuit outputs an amplified differential voltage value obtained by amplifying a differential voltage value between a first voltage value of a battery and a reference voltage value, and outputs an amplified differential voltage value obtained by amplifying a differential voltage value between a second voltage value of the battery and the reference voltage value. A CPU calculates an internal resistance value of the battery based on the amplified differential voltage value outputted from the amplifier circuit, the amplified differential voltage value outputted from the amplifier circuit, a current value of a constant current adjusted by a constant current circuit, and a current value of a constant current adjusted by the constant current circuit. The reference voltage value is a voltage value between the first voltage value and the second voltage value.