1. A server system for providing a plurality of virtual machines constructed on a physical machine equipped with a physical resource, wherein each of the virtual machines includes: an OS recognizing the physical resource allocated to the virtual machine as a resource in the virtual machine; and a software management unit for securing the resource recognized by the OS and allocating the resource to a plurality of pieces of business application software registered in the virtual machine; wherein the software management unit identifies the plurality of pieces of business application software and allocates the resource to the plurality of pieces of business application software according to a preset policy to allocate the resource; and wherein when the software management unit detects an increase or decrease of the resource allocated to the virtual machine, it changes allocation of the resource to the plurality of pieces of business application software in accordance with an amount of the detected resource increase or decrease.