An instruction execution control device operates a plurality of threads in a simultaneous multi-thread system. And the instruction execution control device has a thread selection circuit (30) which detects a state where an instruction has not been completed for a predetermined period during simultaneous multi-thread operation, and controls such that all the reservation stations (5, 6 and 7) can execute only a predetermined thread. Therefore if an entry that cannot be executed from the reservation stations (5, 6 and 7) exists, execution of an entry in the thread that cannot be executed can be enabled by stopping the execution of the thread which has been executed continuously.