Provided is a computer system, including: at least one of database computers, in which a database management system operates; a storage system for storing a database operated by the database management system; and a management computer for managing the database computer and the storage system, the storage system having a cache memory for storing data temporarily and a physical disk drive for storing the database, in which the management computer changes capacity of the cache memory which is allocated to the database computer where the database management system operates, and instructs the database management system to change an access plan according to the change in capacity of the cache memory allocated to the database computer.