A method is provided for calibrating voltage values and current values detected by a high-frequency measuring device. In a first step, a first parameter is calculated based on impedances calculated when the measuring device is connected to a first set of three reference loads and impedances of the first set of three reference loads. In a second step, plasma processing is carried out with the measuring device connected to a load to be measured, and detected voltage and current values are calibrated using the first parameter, and impedances as viewed from a connection point towards the load side are calculated based on the calibrated voltage and current values. In a third step, three impedances that encompass, when displayed on a Smith chart, a narrower range than a range encompassed by the impedances of the first set of three reference loads are determined, where the narrower range includes the impedances calculated in the second step. In a fourth step, a second parameter is calculated based on impedances calculated when the measuring device is connected to a second set of three reference loads respectively having the three impedances and also on impedances of the second set of three reference loads. In a fifth step, the measuring device is connected to the load to be measured, and detected voltage and current values are calibrated using the first parameter and the second parameter.