A portable terminal device includes a supplying unit for supplying, to a CPR, an operating frequency of a clock signal used to operate the CPU, a setting unit for setting one clock level out of a plurality of clock levels assigned with the operating frequency in accordance with an operating state of the CPU and changing an operating frequency stepwise at the clock levels to set the clock level of the operating frequency, a control unit for controlling the operating frequency supplied to the CPU at the clock levels based on settings made by the setting unit, and an input accepting unit for accepting a key input. If the input accepting unit accepts the key input, the setting unit sets the clock level to a predetermined level irrespective of an operating state of the CPU.