PURPOSE: To correctly actuate a timer function and to realize a highly accurate instantaneous interruption prevention by controlling a switch with the action of a supply voltage supervising circuit and compensating the action of a memory circuit with a back-up power source circuit.

CONSTITUTION: When the impressed voltage of a terminal 7 is instantaneously interrupted or when a switch 1 is off for a short time, the output of a stabilizing power source circuit 2 is reduced, a supervising circuit 9 of the supply voltage is operated, and a switch 8 is turned on. Thus, a voltage is supplied from a back-up power source circuit 6 to a CPU 4 and a power source resetting circuit 3. At such time, for a memory circuit 5, a power source is supplied from the circuit 6, the action is compensated, memory information can be sent to the CPU 4 and a correct timer function is displayed. When the voltage reduction is continued for a long time, the switch 8 is turned off by a time constant circuit in the circuit 9.