A line-of-sight direction determination device includes: a line-symmetric position determiner that determines that the corneal reflection image of a left eye or a right eye is located at a line-symmetric position with respect to the center line of the pupils of the left and right eyes； and a line-of-sight direction determiner that determines, from the line-symmetric position determination result, a line-of-sight direction at a specific position including the installation position of an imaging unit or an irradiation unit at the substantially same position as the imaging unit. The line-of-sight direction determination device determines that the corneal reflection image is line symmetric and, from this line symmetry property, determines a specific line-of-sight direction.
