In a program execution method in which a program to be executed is stored in a central unit and a terminal unit acquires the program from the central unit and executes the program, when the central unit receives an acquisition request from the terminal unit, it creates a load module different from the program, which produces the same computation results and differs in the location where essential information is to be embedded, and transmits the load module to the terminal unit together with the essential information necessary for executing the program. The terminal unit receives the program, stores the program on a memory, and executes the program based on the embedded essential information. This method makes it difficult for malicious third parties to illegally execute the program by reverse analysis, and enhances the security of the load module to be executed.