1. A computer system comprising: one or more host computers configured to transmit an Input/Output (I/O) request according to I/O; and one or more storage apparatuses configured to receive the I/O request transmitted from the host computer and to process the I/O request, the storage apparatus comprising: a first storage device; a memory including a main cache that is a cache memory region configured to temporarily store data that is inputted/outputted to/from the first storage device in response to the received I/O request; and a first control device coupled to the first storage device and the memory, the host computer comprising: a second storage device; and a second control device coupled to the second storage device, wherein an entirety or a part of free space of the second storage device is an external cache that is a region used as a cache memory region outside of the storage apparatus, wherein (A) if the I/O is Write, the I/O request is a Write request, and the first control device is configured to write Write data that is data associated with the Write request, into the main cache, and to write the Write data in the main cache into the first storage device, wherein (B) the first control device is configured to write the Write data in the main cache into the external cache, and wherein (C) if the I/O is Read, the following (c1) and (c2) are performed, (c1) the second control device is configured to determine whether or not Read data as target data of the Read exists in the external cache, and (c2) if a result of the determination in the (c1) is positive, the second control device is configured to read the Read data from the external cache.