A coefficient-of-thermal-expansion measuring device includes temperature control device, optical interferometer, and control device including: an actual data acquiring unit sequentially changing an object's temperature and acquiring actual data measured by the optical interferometer at each temperature; a data set generating unit generating pieces of verification data by setting an order of interference of the actual data within a predetermined range, selecting one piece of verification data at each temperature, and generating data sets each containing the selected piece of verification data at each temperature; and a judging unit deriving approximation functions with different orders from each data set, determining an evaluation index value based on differences of verification data from each approximation function, selecting a candidate data set with the smallest evaluation index value for each approximation function, and determining whether the candidate data set is the same for each approximation function to judge applicability of the candidate data set.