A laser light is projected to a thin film deposited on a transparent substrate, and measurement is performed on the entire measurement area of the substrate, and transmission intensity is measured by a transmission light intensity monitor and reflection light intensity is measured by a reflection light intensity monitor at the same points and at the same number of points on the substrate. From the value of "A=1-(R+T)" where R represents reflectivity and T is transmissivity, film thickness is measured and evaluated from the relation of the value A with film thickness. By this procedure, film thickness can be determined on 10,000 substrates or more per minute and film thickness of thin film can be measured over the entire substrate surface.