This intermediate code execution system has a first subsystem having a first interpreter which corresponds to an instruction set generated during compilation and sequentially interprets and executes instructions included in an intermediate code, a second subsystem having a preprocessing section which applies to the intermediate code preprocessing to substitute an instruction pattern consisting of a plurality of instructions with an alternative instruction, and a second interpreter which corresponds to an instruction set including the alternative instruction and sequentially interprets and executes an instruction code included in the preprocessed intermediate code, and a method analysis section which selects either processing to execute the intermediate code by the first interpreter or processing to apply preprocessing to the intermediate code by the preprocessing section and then execute the intermediate code by the second interpreter.