PURPOSE: To improve the versatile applicability of an emulator by using the emulator in common between custom microcomputers belonging to the same family whose quality is developed by changing the peripheral functions based mainly on an existing microcomputer.

CONSTITUTION: An adaptor board 2 mounting a target microcomputer 11, a 1st interface part 24 and a 2nd interface part 25 is used. Thereby, the emulator 3 is used in common between the custom microcomputers belonging to the same family whose quality is developed by changing the peripheral functions of the existing microcomputer, e.g. a network processing unit(NPU) 12 as a core. Consequently, the versatile applicability of the emulator 3 for the custom microcomputers can be improved.