1. An access control method for a computer system in which a plurality of clusters share a storage unit, comprising: predefining an access instruction with exclusive right in addition to an access instruction that is issued with respect to the storage unit from the plurality of clusters; and monitoring, in the storage unit, based on the access instruction with exclusive right transferred from an arbitrary cluster, an access state of an other cluster and executing access instructions with exclusion if a region accessed by an access instruction from the other cluster overlaps a region accessed by the access instruction with exclusive right.