The invention provides a temperature estimation method for a high-temperature member, a metastable tetragonal phase content measurement method, and a degradation determination method. In the temperature estimation method, the metastable tetragonal phase content included in a coating layer formed on the surface of a high-temperature member is measured using X-ray diffractometry, or Rietveld analysis, Raman spectroscopy, and the like, and the surface temperature of the high-temperature member is estimated on the basis of the measured metastable tetragonal phase content.