The contact angle between liquid Si and SiO2 was measured with the sessile drop method at 1723 K. The contact angle changed very unusually due to the evolution of SiO gas at the solid/liquid interface. It was found that the real contact angle between liquid Si and SiO2 is about 80 degrees or less at 1723 K though the apparent contact angle of 95 degrees was observed for a long time during the experiment. The difference in the contact angle can be explained with a model of a composite material. Although the real contact angle is more important in a physical point of view, the apparent contact angle should be adopted in some cases such as simulation works.