A baseline setting method for a measured curve plotted on a reference axis X and a measured value axis Y extending in a direction orthogonal to the reference axis X, the measured curve having a single measured value y i identified with respect to a point x i on the reference axis X, the baseline setting method comprising the steps of: specifying a semicircle or semi-ellipse C xi centered on the reference axis X and an X radius R, expressed by x 2 + a 2 y 2 = r 2 y ≥ 0 f Xi x = r 2 − x 2 1 / 2 / a if peaks of the measured curve are seen in the positive Y direction; comparing individual points (x i - r, f Xi (x i - r)) to (x i + r, f Xi (x i + r)) of the semicircle or semi-ellipse C Xi centered at (x i, 0) with the corresponding points (x i - r, Y Xi-r) to (x 1 + r, y Xi+r) on the measured curve to calculate l Xi − r = y Xi − r − f Xi x i − r: l Xi + r = y Xi + r − f Xi x i + r and to specify the minimum value of l Xi-r to l Xi+r as l (Xi)min; moving the centered position (x i, 0) of the semicircle or semi-ellipse C xi by a distance d parallel to the axis X within a range including point (Xi, 0); recalculating the minimum value of l (Xi+d)min at the centered position (x i +d, 0) of the semicircle or semi-ellipse C Xi; repeating the steps of moving and recalculating while moving the figure within the range of (Xi-r, 0) to (Xi+r, 0); specifying the maximum value of l x i − r min + f Xi − r x i: l x i + r min + f Xi + r x i obtained with respect to each of semicircles or semi-ellipses C Xi-r to C Xi+r centered at (x i - r, 0) to (x i + r, 0), as L( xi); setting a baseline point (x i, L (xi)) corresponding to a specific point (x i, y i) on the measured curve; and connecting the baseline points corresponding to the individual points on the measured curve, to form a baseline. specifying a semicircle or semi-ellipse C xi centered on the reference axis X and an X radius R, expressed by x 2 + a 2 y 2 = r 2 y ≥ 0 f Xi x = r 2 − x 2 1 / 2 / a if peaks of the measured curve are seen in the positive Y direction; comparing individual points (x i - r, f Xi (x i - r)) to (x i + r, f Xi (x i + r)) of the semicircle or semi-ellipse C Xi centered at (x i, 0) with the corresponding points (x i - r, Y Xi-r) to (x 1 + r, y Xi+r) on the measured curve to calculate l Xi − r = y Xi − r − f Xi x i − r: l Xi + r = y Xi + r − f Xi x i + r and to specify the minimum value of l Xi-r to l Xi+r as l (Xi)min; moving the centered position (x i, 0) of the semicircle or semi-ellipse C xi by a distance d parallel to the axis X within a range including point (Xi, 0); recalculating the minimum value of l (Xi+d)min at the centered position (x i +d, 0) of the semicircle or semi-ellipse C Xi; repeating the steps of moving and recalculating while moving the figure within the range of (Xi-r, 0) to (Xi+r, 0); specifying the maximum value of l x i − r min + f Xi − r x i: l x i + r min + f Xi + r x i obtained with respect to each of semicircles or semi-ellipses C Xi-r to C Xi+r centered at (x i - r, 0) to (x i + r, 0), as L( xi); setting a baseline point (x i, L (xi)) corresponding to a specific point (x i, y i) on the measured curve; and connecting the baseline points corresponding to the individual points on the measured curve, to form a baseline.