An apparatus is provided that measures a difference between angles of a subjects neck in first and second periods of time. The apparatus includes a radiating section fitted to a head of the subject, an image pickup section, which picks up an image of an irradiation plane including an illuminated point based on the radiating section, and a computer. Based on image groups obtained by the image pickup section, the computer computes coordinates of the averages of illuminated points in the first and second periods, respectively. The computer computes the distance between a base point on the plane and each of a pair of perpendicular lines, which each extend orthogonally to a horizontal line included in the plane and further drawn to pass onto the base point, and extend respectively from points of the coordinates of the two averages. The computer computes the neck-angle difference based on the distances.
