A controller reads a current value from a current sensor at regular intervals and calculates the average power consumption (Pavg) for a specified length of time (T). When the average power consumption (Pavg) exceeds an upper value (Pu) of power consumption, the controller performs throttling control of a CPU at a throttling rate of r %. If, nevertheless, the average power consumption (Pavg) is above the upper value (Pu), the controller sequentially increases the throttling rate in steps of r %. When the average power consumption (Pavg) is reduced below a lower value (Pl) as a result of the throttling control, the controller sequentially decreases the throttling rate in steps of r % until the upper value (Pu) is reached.