An adjustment method is able to effectively utilize a useless CPU power of a difference between a total CPU power required by a user and a CPU power of an installed computer. In a virtual machine system in which a plurality of virtual machines are operated on a physical machine, a total CPU power which results from totalizing CPU powers required by respective virtual machines is arbitrarily set within a CPU power of the physical machine, a CPU service rate is determined in such a manner that each virtual machine is operated within the thus set total CPU power and a CPU power of a difference between the CPU power of the physical machine and the total CPU power is assigned to a maintenance virtual machine.