Abstract: Commands are classified for each of conventional priority values. It is checked if there is any issue limitation between commands having the same priority value. For command group having issue limitation, it is checked if there is any delay due to the issue limitation. Reverse priority is calculated from an optimizing target group consisting of the commands having a delay due to the issue limitation to a neck command that is a common precedent command of the commands within the optimizing target group, and based upon the reverse priority values, priority calculations are again executed, thereby carrying out a slot mapping process.