PURPOSE: To readily confirm a capacity of a cell and safely enable use of the cell to the limits of its capacity.

CONSTITUTION: In a state, in which ignition and extinguish switches 906, 933, 942 are made on, a cell voltage monitoring circuit 910 resets a microcomputer 907 and lights up a cell figure 924 when a detected voltage E is less than 3.5 volts, suspends operation of the microcomputer 907, lights up the cell figure 924 and continuously rings a buzzer 925 when the detected voltage E is not less than 3.5 volts but less than 4 volts, and flickers the cell figure 924 and continuously rings the buzzer 925 when the detected voltage E is not less than 4 volts but less than 4.2 volts. Further, the cell voltage monitoring circuit 910 puts out the cell figure 924 and suspends the buzzer 925 when the detected voltage E is not less than 4.2 volts.