1. A method for smoothing a thermal image produced based on thermal image values of a work to be measured on which a black body coating is applied, comprising: a removal step for removing thermal image values which are smaller than an estimated value from the thermal image values of the work as unexpected values, wherein the removal step comprises: a first selection step for selecting maximum points from the thermal image values; a second selection step for selecting additional points having bigger thermal image values than the estimated value, which is set on a line between each two of the adjacent maximum points selected in the first selection step; and an interpolation step for performing a linear interpolation using the maximum points selected in the first selection step and the additional points selected in the second selection step.