1. A server system comprising: a plurality of servers each having one or more virtual machines being able to move to the other servers; and a management unit which manages the movement of one or more virtual machines on one of said servers to another of said servers based on first information showing electric power data and a first condition that the number of servers in operation is reduced as much as possible, and stops supplying electric power to the server having no virtual machine in operation as a result of said movement, wherein said first information includes information indicating real time electric power consumption in each of said one or more virtual machines and information indicating real time remaining electric power in each of said servers, said real time remaining electric power in said each of said servers being calculated by extracting real time electric power consumption in said one or more virtual machines on said each of said servers and real time electric power consumption excluding said real time electric power consumption in said one or more virtual machines on said each of said servers from maximum electric power for said each of said servers, wherein said management unit includes a control unit which searches for an optimal assignment of virtual machines to said servers on said first condition and a second condition that, for each of said servers, said real time remaining electric power in said each of said servers is greater than or equal to said real time electric power consumption in one or more virtual machines moved to said each of said servers, and controls the start and stop of said servers.