Validity of a line-of-sight direction detected by a gaze point detection unit is determined by using a positional difference that is a calibration value used for detecting a position of a corneal curvature center. Therefore, it is possible to easily and effectively determine the validity of the line-of-sight direction without additionally using a system or the like for detecting the validity of the line-of-sight direction. Consequently, it is possible to accurately detect line-of-sight directions of various subjects, such as a subject whose left and right eyeballs have different corneal curvature radii or a subject whose line-of-sight directions of left and right eyeballs are largely different due to the influence of strabismus or the like.