A synchronous reference code indicative of the fact that synchronous updating was made is provided to data which is to be applied to a critical section, and the code is set when synchronous updating is made. After a sentence in the critical section is executed, it is confirmed whether or not the synchronous updating of the data was made. In a thread for synchronous reference, reference is made, it is confirmed whether or not synchronous updating was made, and then the correctness of the updating is confirmed. When the synchronous updating is not made, the execution of the critical section is completed. Thereby the simultaneous execution of the critical sections and reduction of an overhead are realized.