A method of monitoring a virtual computer system including a network device coupled to a client computer, sever computers each coupled to the network device, and a performance degradation source locator apparatus coupled to the network device and the server computer, obtaining, by the performance degradation source locator apparatus, performance information on physical computer resources and performance information on virtual computer resources of extracted virtual machines and server computers and the virtual machine and the server computer in which the latency has increased to locate a part in which the latency has increased.