Thirty-four subjects with constant exotropia were included. Two independent ophthalmologists measured the angle of ocular deviation using alternate prism cover test (APCT). The video files and data with the changes in ocular deviation during the alternate cover test were obtained using video-oculography (VOG). To verify the accuracy of VOG, Investigator compared the value obtained using VOG and the angle of a rotating model eye, and subsequently made a new linear equation using these data. The calculated values obtained using VOG were compared with those obtained using APCT to determine the diagnostic accuracy of VOG.