1. A method of measuring sizes of objective portions of a circuit of a ceramic material provided with an organic thin-film circuit pattern thereon, said method comprising the steps of: obtaining a first image of the circuit by illuminating the circuit with linearly polarized illuminating light and detecting reflected light having a direction of vibration perpendicular to that of the linearly polarized illuminating light; obtaining a second image of the circuit by illuminating the circuit with monochromatic illuminating light having a short wavelength and detecting fluorescence produced by the circuit and having a wavelength longer than that of the monochromatic illuminating light; selecting either the first image or the second image as the image of objective portions of the circuit; calculating positions of the objective portions in the selected image; converting coordinates of the positions of the objective portions in the selected image into coordinates of an absolute coordinate system based on a relation between the coordinates of the objective portions in the selected image and coordinates of the absolute coordinate system; and determining a size of an arbitrary objective portion by calculating a difference between coordinates in the absolute coordinate system of two objective portions.