There is provision of a method of determining wastage including: processing a substrate using a plasma generated by multiple gases including fluorine gas; obtaining light emission intensity of each gas of the multiple gases including fluorine gas from the plasma, by an optical emission spectrometer (OES); and calculating a wastage rate of a particular expendable part from the obtained light emission intensity of each gas of the multiple gases including fluorine gas, with reference to a storage section storing a wastage rate of the particular expendable part in association with the light emission intensity of each gas of the multiple gases including fluorine gas.