A battery monitoring device comprising: a battery monitoring circuit including a battery to be monitored; a constant current circuit that is provided in the battery monitoring circuit and adjusts a current of the battery flowing in the battery monitoring circuit to a constant current of a first current value (Idis1) and a constant current of a second current value (Idis2) which is a current value different from the first current value (Idis1); an amplifier circuit that outputs a first amplified differential voltage value obtained by amplifying a differential voltage value between a first voltage value (Vbat1) of the battery and a predetermined reference voltage value (Vdac) when the constant current of the first current value (Idis1) adjusted by the constant current circuit flows in the battery monitoring circuit, and outputs a second amplified differential voltage value obtained by amplifying a differential voltage value between a second voltage value (Vbat2) of the battery and the reference voltage value (Vdac) when the constant current of the second current value (Idis2) adjusted by the constant current circuit flows in the battery monitoring circuit; and a controller that calculates an internal resistance value (Ri) of the battery based on the first amplified differential voltage value outputted by the amplifier circuit, the second amplified differential voltage value outputted by the amplifier circuit, the first current value (Idis1) of the constant current adjusted by the constant current circuit, and the second current value (Idis2) of the constant current adjusted by the constant current circuit, wherein the reference voltage value (Vdac) is a voltage value between the first voltage value (Vbat1) and the second voltage value (Vbat2). a battery monitoring circuit including a battery to be monitored; a constant current circuit that is provided in the battery monitoring circuit and adjusts a current of the battery flowing in the battery monitoring circuit to a constant current of a first current value (Idis1) and a constant current of a second current value (Idis2) which is a current value different from the first current value (Idis1); an amplifier circuit that outputs a first amplified differential voltage value obtained by amplifying a differential voltage value between a first voltage value (Vbat1) of the battery and a predetermined reference voltage value (Vdac) when the constant current of the first current value (Idis1) adjusted by the constant current circuit flows in the battery monitoring circuit, and outputs a second amplified differential voltage value obtained by amplifying a differential voltage value between a second voltage value (Vbat2) of the battery and the reference voltage value (Vdac) when the constant current of the second current value (Idis2) adjusted by the constant current circuit flows in the battery monitoring circuit; and a controller that calculates an internal resistance value (Ri) of the battery based on the first amplified differential voltage value outputted by the amplifier circuit, the second amplified differential voltage value outputted by the amplifier circuit, the first current value (Idis1) of the constant current adjusted by the constant current circuit, and the second current value (Idis2) of the constant current adjusted by the constant current circuit, wherein the reference voltage value (Vdac) is a voltage value between the first voltage value (Vbat1) and the second voltage value (Vbat2).