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; storing, in a register within the storage unit, a table rewrite instruction that is transferred from at least one of the plurality of clusters and instructs rewriting of the table; and determining, in the storage unit, a priority order of an access instruction transferred from an arbitrary cluster and an access instruction transferred from another cluster, wherein the determining waits for the access instruction transferred from the other cluster to end when the access instruction transferred from the arbitrary cluster is an access instruction with exclusive right and accesses an address that overlaps an address of an access instruction that is being executed, and wherein the determining waits for execution of the table rewrite instruction stored in the register to end when the access instruction transferred from the arbitrary cluster is an access instruction with exclusive right and accesses an address that does not overlap an address of an access instruction that is being executed but overlaps an address of the table rewrite instruction.