1. A swallowing function evaluating apparatus comprising: a height variation detecting unit configured to detect a variation in a height of a position of a surface of a throat, without exerting a force to the surface of the throat, in a height direction in which a thyroid protrudes; and an analyzing unit configured to obtain an index for evaluating a swallowing function, based on the variation, wherein the height variation detecting unit is configured to detect variations in a height at a plurality of points in a thyroid moving direction in which the thyroid moves during a swallowing motion and in a widthwise direction that is orthogonal to both the thyroid moving direction and the height direction, wherein the plurality of points are arranged in a plurality of rows, each row being arranged along the widthwise direction, wherein the analyzing unit is configured to: add up the variations in the height of at least 4 points in each row of points respectively to obtain an added up value for each row, the plurality of rows being arranged with a predetermined interval in the thyroid moving direction on the surface of the thyroid; obtain the index using the added up values, each of the added up values corresponding to a respective row of the plurality of rows: separately add up the variations in height in a protruding direction in which the thyroid protrudes and in an opposite direction opposite to the protruding direction to obtain separately added up values; obtain, for each of the protruding direction and the opposite direction, a distribution of the added up values in the thyroid moving direction when the separately added up values are viewed in the thyroid moving direction, and obtain, as the index, at least one of a height direction component deviation distance in the height direction between positions, each of the positions indicating a bias of a respective distribution of the distributions of the protruding direction and the opposite direction, and a thyroid moving direction component deviation distance in the thyroid moving direction between the positions.