In a parallel processor system for executing a plurality of threads in parallel to each other by a plurality of thread execution units, the respective thread execution units allow for forking of a slave thread from an individual thread execution unit into another arbitrary thread execution unit. The respective thread execution units are managed in three states, a free state where fork is possible, a busy state where a thread is being executed, and a term state where a thread being terminated and yet to be settled exists. At the time of forking of a new thread, when there exists no thread execution unit at the free state, a thread that the thread execution unit at the term state has is merged into its immediately succeeding slave thread to bring the thread execution unit in question to the free state and conduct forking of a new thread.