In a computer system including physical servers on which virtual servers for business use operate and a management server communicably connected to the physical servers, the management server requests physical servers that are targets of performance measurement to activate a performance measurement virtual server based on a virtual server image including an operating system and a performance measurement program. Performance information measured by the performance measurement virtual server activated on each physical server is fed to the management server. It is therefore possible in the communication system to measure a margin processing performance of each physical server according to the same criterion.