Provided are a method and an apparatus for monitoring performance of a virtual computer. In a method of controlling a computer system including a computer, the computer executes a virtualization program for causing logically divided resources of the computer to operate as first and second virtual computers, the first virtual computer executes a first OS, and the second virtual computer executes a second OS. In the method, information regarding the resources allocated to the first virtual computer and the second virtual computer by the virtualization program is obtained from the virtualization program, information indicating performance of the first virtual computer is obtained from the first OS, information indicating performance of the second virtual computer is obtained from the second OS, the obtained information and information indicating a time of obtainment of the information are stored in a storage system, and stored information is output.