A coefficient-of-thermal-expansion measuring device includes: a temperature detector detecting an object's temperature; an optical interferometer measuring an object's length using single-wavelength light; an actual data acquiring unit changing the object's temperature to temperatures and acquiring actual data of the length measured by the optical interferometer at each temperature; a data set generating unit generating pieces of verification data for each actual data by setting an order of interference 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, the data sets having different combinations of selection of the pieces of verification data; and a judging unit calculating a linear approximation function for each data set, and judging, using an evaluation index value based on differences of the verification data in each data set from the linear approximation function, applicability of each data set.