1. A simulator for simulating, on a host computer having a host central processing unit, a target program which is prepared for execution on a target computer and comprises a typical process part representing a typical process which is commonly executed by a plurality of programs and an atypical process part representing an atypical process which is peculiar to the target program, said simulator comprising: typical-process executing means for executing said typical process in accordance with at least an instruction of said host central processing unit other than instructions of said target computer; atypical-process executing means for interpreting said target program to execute said atypical process in accordance with said atypical process part; and control means, connected to said typical-process executing means and said atypical-process executing means, for controlling operations of said typical-process executing means and said atypical-process executing means.