PURPOSE: To effectively utilize a battery by measuring a battery voltage which is being discharged at a prescribed time interval, deriving a voltage drop rate of the battery from voltages of its two points and time, and predicting the time required for reaching a discharge end voltage therefrom.

CONSTITUTION: A voltage of a battery 2 for supplying power to a load 3 is detected by a voltage detecting circuit 11 and outputted to a voltage drop rate arithmetic circuit 13. The voltage drop rate arithmetic circuit 13 calculates a voltage drop rate from battery voltages of two points of some time point and after a prescribed time has elapsed and a clock pulse of a clock oscillating circuit 12. Also, by a discharge float arithmetic circuit 16, the time to a discharge end voltage is calculated from said result, the clock pulse from the clock oscillator 12, and a discharge end voltage which has been set by a voltage setting device 14, and its result is displayed on a float indicator 17. In such a way, the battery 2 can be utilized without being damaged.