To monitor discharge current accurately, even if the discharge current changes transiently.

In a battery state monitoring circuit 300 for monitoring the discharge current and displaying the residual capacity of a battery, a low-pass filter 258 is added to a current sensing terminal of the circuit 300. By controlling the cut-off frequency of the low-pass filter 258 according to the discharge current by means of a microcomputer 261, the discharge current is monitored accurately, and thereby the accuracy of displaying the residual capacity of a battery is increased.