A computer system for managing a plurality of virtual machines, the computer system including: a processor; and a memory coupled to the processor, wherein the processor executes a process includes: recording, on the memory, an operation history of a virtual machine that is related to a user operation in the plurality of virtual machines; determining whether the user operation is performed or not at switching of the virtual machines by referring to the operation history and comparing an operation time of the virtual machine that has operated most recently with the operation time of a control table, the control table being stored in the memory; and increasing an operation frequency of a CPU when performance of the user operation is detected.