PURPOSE: To know the residual use time of a battery, by a method wherein not only the life of the battery but also the use residual time of the voltage thereof are displayed on a display device when the lowering in the voltage of the battery for a power source is detected by a voltage detection circuit.

CONSTITUTION: When a user operates a mode switch MS for the sake of diving, since a control signal PC is brought to an H-level one, a power source control circuit 12 operates to supply a current to a depth-sounding apparatus circuit 8. Therefore, the depth-sounding apparatus circuit 8 is brought to an operation state to output depth-of-water information PS and the depth of water is displayed on a display device 7. When the voltage of a battery BT for a power source is lowered during the diving of the user, a detection signal SK of an H-level is outputted from a voltage detection circuit 15. Therefore, the resetting of a timer circuit 16 is released and the frequency dividing signal PF from a frequency dividing circuit 3 is inputted to the time circuit 16 and subtraction counting operation is started from a definite battery use residual time to output timer information PT. By this method, the remaining diving permitting time can be known.