In recent years, ophthalmic problems such as asthenopia and strabismus due to watching videos on smartphones have increased, particularly among the younger generation. A smartphone can be operated with one hand regardless of posture. Consequently it is possible to use a smartphone at a closer distance than the usual near-sighted working distance (40-30 cm) for long periods. This may be the cause of the problems described above. In this study we aim to investigate the control of eye movements during viewing of a video on a smartphone. The video features intense two-dimensional images with depth information. The gaze of both eyes was measured, and the convergence distance was examined. Six university students participated in the study. They were asked to watch a 15-minute video on a smartphone, during which their eye movements were measured. During the experiment, the participants watched a self-made “video moving through a 3-D maze.” For each viewing distance, the convergence distance was calculated based on the intersection of the eyes’ gaze. In some instances, the viewing distance and the convergence distance did not match when watching the video, suggesting that the mismatch could lead to eye strain and strabismus.