An electric power control method is provided. In the method, a range having a maximum value, which is an output ratio of initial target signal to disturbance wave added with an offset addition value, and a minimum value, which is the output ratio of initial target signal to disturbance wave subtracted an offset subtraction value, is set. If an output ratio of target signal to disturbance wave is equal to or more than the maximum, the output ratio of target signal to disturbance wave is set to the maximum, if an output ratio of target signal to disturbance wave is equal to or less than the minimum, the output ratio of target signal to disturbance wave is set to the minimum, and if an output ratio of target signal to disturbance wave is within the range, no adjustment is performed.