An rf sputtering apparatus was equipped with an optical spectrometer with which the characteristic lines of sputtered species were observed. A specially designed CaO disk or a thin CaO layer on a quartz plate were used as the targets. It was observed that the intensity of the calcium peak increased more rapidly than the thicknesses of the films obtained as a function of the argon gas pressure. This was assumed to be due to the resputtering of calcium oxide from the substrate. The effect of the substrate temperature on the film thickness was also studied. The temperature was changed from 100 to 350 °C. No change in the film thickness was observed.