The number of executed instructions (I.sub.u) in a user mode as one of performance indexes of a computer, and the total number of executed instructions (I.sub.t) as one of power consumption indexes are used. These parameters have a relationship of E-I.sub.t /I.sub.u with energy index E. An operating speed (operating frequency) of CPU is increased or decreased so that a value of the performance index may be increased within a range of the power consumption index specified by a user, or a value of the energy index E may be decreased. By doing so, power management processing may be performed, which balances the power saving with the performance.