A program is divided into several instruction streams, and each of them is executed as a thread. A thread processor executed the thread. The thread generates another thread, but one thread is controlled to make a fork operation at most once. Each thread is terminated in the order of generations. A thread manager may be shared with the several thread processors or be distributed to the several thread processors. The thread manager includes a thread sequencer and a thread status table. The thread status table manages execution status of each thread processor and parent-child relation. The thread sequencer requests a thread generation and permits its termination in accordance with the content of the thread status table. The thread processor can execute a thread speculatively.