In the data processor in which a combination of multiple specific instructions is prohibited, an instruction set is employed that additionally defines that prohibition combination pattern as a separate instruction. With respect to the prohibition combination pattern additionally defined as the separate instruction, for example, in order to make a definition in such a manner that an instruction dispatch mechanism for the instruction set that is present before the additional definition is used as is, the instruction to be additionally defined by the prohibition combination pattern is limited to an instruction type that is the same as the instruction defined only with a latter-half code of the instruction in a case of an instruction set in which the instruction set that is present before the additional definition includes a prefix code.