In this study, we discuss indexes that can be used to quantify users' visual fatigue when they watch three-dimensional (3D) visual contents with consumer displays. We discuss two indexes with values that can be calculated by biological signal processing without disturbing the viewing of 3D contents. One is a video-oculography (VOG) index, and the other is an electro-oculography (EOG) index. To clarify the effectiveness of these indexes, we conducted simple experiments. Our results confirmed the relationship between these biological indexes and the users' visual fatigue.