In a multi-thread executing method of dividing a single program into a plurality of threads and executing the program by a plurality of processors in parallel, at a time of every fork instruction of the executing thread, when there already exists a child thread generated form the above thread, the program cancels the child thread or makes invalid all the fork instructions other than the first fork instruction having succeeded in forking the child thread, hence to select one fork instruction for creating an effective child thread from a plurality of fork instructions existing within a parent thread, during the execution of the parent thread. Therefore, it can assure the Fork-Once limitation at a time of the program execution.