Optical image data is acquired by irradiating a pattern with light emitted from a light source. A threshold value is specified by internally dividing a minimum value and a maximum value of a signal amount of reference image data by a division ratio. A position corresponding to a signal amount of a threshold value is determined as an edge of a pattern of the reference image data. A position of a signal amount equal to the threshold value is determined as an edge of the pattern of the optical image data. A line width error is obtained as a difference between a first line width of the optical image data and a second line width of the reference image data. A new threshold value is specified in the case of fluctuation of a light quantity of the light source or decrease of a contrast value of the optical image data.