PURPOSE: To protect data at the power supply interruption, by inhibiting the access of an external memory through the output of a flip-flop which is set before the interruption of a power supply for a microcomputer and reset after the microcomputer is reset after the application of power supply.

CONSTITUTION: Present invention refers to a microcomputer which protects data stored in an external memory 6 through uncertain operation of a microcomputer 1 accompanied with the interruption and application of a power supply 7. Further, a flip-flop FF16 which is set before the power supply 7 for the microprocessor 1 is interrupted and reset after said microcomputer 1 is reset after the application of the power supply 7 is provided for said microcomputer, so that the access of the external memory 6 can be inhibited with the output of said FF16. That is, since an operation unguarantee period T of the microcomputer 1 is within an access inhibiting ("H" level) period of the external memory 6, the data of the external memory 6 can not be destroyed at the interruption and application of the power supply.