An object of the present invention is to provide a storage system (10) which is shared by a plurality of application programs (AP#O...AP#N), wherein optimum performance tuning for a cache memory (25, 35) can be performed for each of the individual application programs. The storage system (10) of the present invention comprises a storage device (40) which provides a plurality of logical volumes (LU#O...LU#N) which can be accessed from a plurality of application programs (AP#O..AP#N), a controller (20) for controlling input and output of data to and from the logical volumes (LU#O...LU#N) in response to input/output requests from the plurality of application programs (AP#0...AP#N), and a cache memory (25,35) for temporarily storing data input to and output from the logical volume, wherein the cache memory (25, 35) is logically divided into a plurality of partitions (PO1, PO2, PO3, P11, P12, P13) which are exclusively assigned to the plurality of logical volumes respectively.