PURPOSE: To suppose a voltage drop to a low value by using amplifying means for amplifying the drop generated at an element for detecting a battery current and to simultaneously prevent an erroneous calculation due to an offset voltage of the means.

CONSTITUTION: When a charging/discharging current flows to a battery 1, voltage drops V1, V2 are generated at a resistor 2 of current detecting means. In this case, since an inverting amplifier 4 generates a positive voltage proportional to the V1 and a non-inverting amplifier 5 generates a positive voltage proportional to the V2, a calculation with the positive values can be performed. On the other hand, when no charging/discharging current flows to the battery 1, they are not input to the amplifiers 4, 5, but a superposing voltage generator 6 always generates a negative voltage, which is divided, and input to the amplifiers 4, 5. Accordingly, the output voltages of the amplifiers 4, 5 become positive values. A calculator 10 stores a calculated result when no charging/ discharging current flows to the battery 1, subtracts the stored calculated result from the calculated result when the current flows, integrates the result, and displays it on a display 11. Accordingly, an offset voltage when the current does not flow to the battery 1 is not erroneously calculated.