1. A microcomputer having an on-chip debugging function, comprising: a central processing unit for executing instructions; and a first monitor section which performs data transfer to and from a second monitor section, determines a primitive command to be executed based on the receive data from said second monitor section, and performs processing for execution of the determined primitive command, a monitor program for executing a processing of said first monitor section is stored in a ROM, said second monitor section being provided outside said microcomputer and performing a processing to convert a debugging command into at least one primitive command in order to reduce the size of an instruction code for realizing the first monitor section or a scale of the first monitor section.