A method for calibrating a high frequency measuring device so as to accurately measure plasma processing parameters within a chamber. A calibration parameter is calculated from a first set of three reference loads measured by a high frequency measurement device. A second calibration parameter is calculated from S parameters measured between a connection point where the high-frequency measuring device is connected and the inside of the chamber of a plasma processing device. A second set of three reference loads, which include the impedance previously calculated and encompass a range narrower than that encompassed by the first set of three reference loads, is measured with the reference loads in the chamber. Another calibration parameter is calculated from the measured impedances of the second set of three reference loads measured by the high- frequency measuring device, and the true values of those impedances, and a detected voltage signal and a detected current signal are calibrated using the above calibration parameters.