A microcomputer which is to be utilized in a condition of being built into an apparatus such as a vehicle ECU, includes a ROM having stored therein a program which is prepared by a user and a boot ROM which stores an initialization program for performing at least essential initialization processing which is required for all application programs that will be executed by the microcomputer. The need for the user to generate program contents relating to such initialization processing is thereby eliminated, while increased reliability of operation of the microcomputer is achieved. In addition, it is possible for user-specified initialization data to be read out from a user program and utilized in a part of the initialization processing of the initialization program.