PURPOSE: To inform of infeasibility of write due to either the reduction of a battery voltage or another factor other than that by judging whether or not data is written on nonvolatile memory normally, and detecting the voltage of a battery when it is judged that write is infeasible.

CONSTITUTION: When the infeasibility of write of the data occurs even by the repeat of a write operation on an EEPROM 7, battery voltage detecting processing to detect the voltage of a lithium battery 9 is executed. In other words, a battery voltage detecting part 10 outputs a detection signal (b) when the driving voltage of the EEPROM 7 outputted from a power source circuit 8 goes less than a rated voltage 2V based on the voltage of the lithium battery 9. As a result, a message representing the infeasibility of write due to the reduction of the voltage is displayed on the dot matrix display part of an LCD 2 in display processing. Thereby, it is possible to inform of the infeasibility of write due to either the reduction of the battery voltage or the factor other than that to a user.