A gear positioning device according to an embodiment includes: a chuck configured to hold a gear; a rotation drive mechanism configured to rotationally drive the chuck so that the gear rotates around a predetermined rotation axis; a displacement meter configured to continuously or periodically acquire a measurement value representing a distance between a reference point located outside the gear and an outer peripheral surface of the gear while rotating the gear; and a control device configured to set a part of the outer peripheral surface of the gear as a measurement object on the basis of a rotation angle of the gear, the measurement value, and at least one predetermined reference value and control the rotation drive mechanism so that the measurement object is disposed at a reference position.