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 local maximum points whose thermal image values are maximal 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 local maximum points selected in the first selection step; and an interpolation step for replacing the thermal image values between the local maximum points selected in the first selection step and the additional points selected in the second selection step, removed as the unexpected values in the removal step, by performing a linear interpolation using the local maximum points selected in the first selection step and the additional points selected in the second selection step.