A method and device to accurately obtain very small quantity of wear of the order of nanometers of a protective film on the surface of a sliding member. A quantity of wear on the surface of a measurement sample including a base and a coating layer is measured by making a spectrum of the surface elements in a reference sample using a surface-element analysis device which analyzes elements on the surface of a substance from an energy spectrum of charged particles obtained by applying excited ionization radiation on the reference sample equivalent to the measurement and by measuring charged particles generated from the surface of the substance. A step of obtaining signal intensity ratios of plural elements from the spectrum is repeated a plurality of times while the surface of the reference sample is being etched and calibration curves which indicate a distribution of the signal intensity ratios of the plural elements in the reference sample are made. Subsequently, an energy spectrum of charged particles from the surface of the measurement sample is measured, signal intensity ratios of specific elements are calculated and compared with the calibration curves to determine a quantity of wear of the measurement sample.