A plurality of register windows in a multithread processor are each provided for a corresponding thread and capable of storing data to be used for instruction processing in an arithmetic unit. A work register in the multithread processor is capable of mutually transferring data with respect to the register windows and the arithmetic unit. A multithread control unit in the multithread processor controls data transfer among the register windows, the work register and the arithmetic unit on the basis of an execution thread identifier identifying the thread to be executed in the arithmetic unit. This enables conducting the multithread processing at a high speed.