On the occasion of returning an error code to an application program to be executed under different execution environments of a computing system, it is requested to fully utilize a system of the existing instruction set to improve the ROM efficiency in the implementation of its common code and to eliminate useless insertion of instruction. For this purpose, in the present invention, a common error code to be returned, from the program to be used to be independent on different OS, to the execution program corresponding to the application program is determined as a value in the range of numerical value to be set with the instruction set of CPU and the defined instruction to load immediate value and thereby the common error code is held within the instruction code of the instruction to load immediate value. Particularly, a value of common error code is determined within the range where the MSB of the setting part of the instruction to load immediate value becomes zero (0).