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 a set of maximum points from the thermal image values, comprising the start point at the minimum position coordinate, the end point at the maximum position coordinate, and the maximum points where the thermal image value is bigger than those of the adjacent points; 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 replacing the thermal image values between the maximum points selected in the first selection step and the additional points selected in the second selection step, removed as the unexpected values, by performing a linear interpolation using the maximum points selected in the first selection step and the additional points selected in the second selection step. 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 a set of maximum points from the thermal image values, comprising the start point at the minimum position coordinate, the end point at the maximum position coordinate, and the maximum points where the thermal image value is bigger than those of the adjacent points; 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 replacing the thermal image values between the maximum points selected in the first selection step and the additional points selected in the second selection step, removed as the unexpected values, by performing a linear interpolation using the maximum points selected in the first selection step and the additional points selected in the second selection step. a first selection step for selecting a set of maximum points from the thermal image values, comprising the start point at the minimum position coordinate, the end point at the maximum position coordinate, and the maximum points where the thermal image value is bigger than those of the adjacent points; 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 replacing the thermal image values between the maximum points selected in the first selection step and the additional points selected in the second selection step, removed as the unexpected values, by performing a linear interpolation using the maximum points selected in the first selection step and the additional points selected in the second selection step.