1. A computer system, comprising: one or more host computers configured to transmit an Input/Output (I/O) request; and one or more storage apparatuses configured to: receive the I/O request transmitted from the host computer; and process the I/O request, each 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 a write, the I/O request is a write request, and the first control device is configured to: Write data associated with the write request into the main cache, and write the data in the main cache into the first storage device, (B) the first control device is configured to write the data in the main cache into the external cache, and (C) if the I/O is a read, the I/O request is a read request; (c1) the second control device is configured to determine whether read data as target data of the read request exists in the external cache, and (c2) if a result of the determination in (c1) is positive, the second control device is configured to read the read data from the external cache; and wherein the first control device is configured to delete data from the first storage device such that a total data size of the first storage device is smaller than a cache use size that is determined on a basis of a working state of the host computer and is used as the external cache.