To provide a computer system, a management computer and a storage system, and a storage area allocation amount controlling method for improving I/O performance of the host computer. In a computer system comprising a storage system comprising one or more storage devices with storage areas, a host computer which uses a storage area of the storage device, and a management computer for dynamically allocating the storage area in response to an input/output request from the host computer; wherein the management computer monitors dynamic allocation of a real storage area to a storage area in the storage system, and calculates allocation increment amount to the allocated storage area based on the allocation frequency and the total amount of allocation.