PURPOSE: To check volatilization of the storage data due to the voltage drop of a short period when the application of a microcomputer is stopped, by providing a check code area and checking the check code by a check program.

CONSTITUTION: A microcomputer 1 contains a microprocessor 1a and a main memory 1b. The memory 1b includes a check code area to store check codes (I), (II) and (III). Then a check program is first executed when a power supply is applied after the application of the computer 1 is started. Thus each check code is checked. When it is confirmed that all check codes have no error, a task is carried out. If those check codes have some errors, a back-up memory is rewritten after deciding that a back-up battery 3 has volatilization or change of data owing to a temporary drop of voltage. Thus it is possible to detect effectively the volatilization of the back-up memory due to the voltage drop of a short period when the application of the computer is discontinued.