PURPOSE: To execute direct reading and check of battery voltage by a simple constitution, by measuring the voltage of a service interruption backup battery by a digital value, comparing it with a set value, and generating an alarm or displaying it by a digital numeric device.

CONSTITUTION: Voltage of a service interruption backup battery 10 of a measuring device passes through contacts 11a, 11b controlled by a relay driving part 3 responding to an input from a microcomputer 100 of every constant interval of time of an analog inputting circuit 13, is inputted to an A/D converting circuit 4 and is measured as a digital value. Subsequently, the voltage is displayed on a display part 8 through the computer 100, and also is compared with a set value written in a memory 7, by which generation of an alarm, etc. by an alarm part 12 in case when the battery voltage drops are controlled through a CPU6, etc. According to this simple constitution by which reference voltage, an operational amplifier, etc. are not used and a digital processing part required for the measurement is used in common, direct reading and check of the backup battery voltage can be executed exactly.