An instruction execution control device operates a plurality of threads in a simultaneous multi-thread system. The device has architecture registers (22-0, 22-1) for each thread, and a selection circuit (32, 24) which, when an operand data required for executing a function is read from a register file (20), selects in advance a thread to be read from the register file (20). This makes it possible to select an architecture register at an early stage, and although the number of circuits in a portion for selecting the architecture registers increases, the wiring amount of the circuits can be decreased, because the architecture register of the thread to be read is selected in advance.