Formation of thin insulating SiO2 films by anodic oxidation of silicon was studied as a part of investigating an alternative method of fabricating low-cost silicon MIS solar cells. Anodization in the constant-voltage mode was carried out in nonaqueous ethylene glycol solution. The film thickness was carefully measured using an ellipsometer of wavelength 6238 . MIS cell performance was evaluated by comparing the open circuit voltage VOC and the short circuit current density ISC with those of the bare Schottky cell (without anodization) under illumination by a tungsten lamp. It was found that anodization in the constant-voltage mode can increase VOC without reducing ISC, and that anodization in the constant-voltage mode is more controllable and reproducible. The optimun formation voltage which gives the maximum VOC of the MIS cell depends on the forming voltage of oxide. A brief discussion on the mechanism for VOC increase is given.