An apparatus for measuring a downward rotation amount of eyeball that measures a length from a distance-vision eye point to a near-vision eye point of a spectacle lens actually worn by a wearer and attached to a frame having an upper side portion and a lower side portion, includes: a line-of-sight position detecting unit that detects a position of a line of sight corresponding to the distance-vision eye point of the wearer and a position of a line of sight corresponding to the near-vision eye point; and a computing unit that computes a distance between the position of the distance-vision eye point and the position of the near-vision eye point detected by the line-of-sight position detecting unit, wherein the line-of-sight position detecting unit has an arm member that is rotatable at one end and positioned at a lateral position of an eyeball of the wearer, a front detecting mechanism that is disposed on the other end side of the arm member and detects a front position of the eyeball of the wearer, and an arm rotation angle detecting unit that detects a rotation angle of the arm member.