PURPOSE: To emulate an emulation object program having loop structure at a high speed.

CONSTITUTION: An address retrieval means 23 retrieves whether or not the execution address of the emulation object program 1 is registered in an address/ machine word instruction code storage table 3 and a machine word instruction code taking-out means 24 takes a machine word instruction code out of the program 1 when the address is not retrieved and out of the table 3 when the address is retrieved. A machine word instruction code converting means 25 converts the instruction code taken out of the program 1 into a machine word instruction code of a host machine and an address/machine word instruction code storage means 26 stores the converted instruction code in the table 3 together with the address. A machine word instruction code executing means 27 executes the instruction code converted by the machine word instruction code converting means 25 or the instruction code taken out of the table 3.