1. A server system constructed on a physical machine equipped with a physical resource including at least a processor and a storage device, wherein the server system comprising a plurality of virtual machines, 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, when executing each program, the plurality of pieces of business application software based on a program boundary which is a component unit or execution unit of a plurality of programs included in the plurality of pieces of business application software and, identifies, if the plurality of programs included in the plurality of pieces of business application software are Java, by using Java's archive unit, wherein the Java's archive unit is based on a class loader unit which is set for each multi-level archive having a hierarchy information that identify class loader after the business application software is executed: allocating the resource to the plurality of pieces of business application software according to a preset policy to allocate the resource; and changing, when detecting an increase or decrease of the resource allocated to the virtual machine, 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.