A response-time-difference calculation/request-number converting means (523) calculates a change of throughput based on a response time monitoring result, a target value of the response time, a quantitative model and performance specification information. A server selection means (524) input the performance specification information in the quantitative model acquired to calculate a throughput of each of pool servers, selects a pool server having a throughput corresponding to a throughput larger than and closest to the change of throughput, and instructs provisioning means (513) to execute configuration modification control on the selected pool server (502). The provisioning means (513) modifies the configuration so that the pool server (502) functions as an application server.