The present invention has an objective of minimizing deterioration of the processing speed of a Java accelerator device even when stack overflow occurs in a stack memory unit. A first thread presently allocated to a first stack area of a stack memory unit 113 to which a fourth thread belongs is saved in a virtual stack area of a main storage medium 103. Thereafter, the data of the fourth thread as stack data to be switched is copied to the first stack area of the stack memory unit 113 by the controller unit 112 (accelerator device 101).