Abstract: A gear measuring apparatus comprises detecting device for outputting detection signals, an X-direction linear travel system for making the detecting device travel linearly in an X direction perpendicular to a radial line and rotation device for rotating the detecting device by causing the X-direction linear travel system to rotate around the rotation center of the gear. The gear measuring apparatus further comprises control device for interlocking the rotation and the X-direction linear travel of the detecting device so that a probe draws the standard involute tooth form, because the detecting device, which are set to an initial position, are rotated at only an extremely small angle .theta. by the rotation device, and at the same time the detecting device are made to travel linearly by .theta..multidot.(Dp.multidot.cos.alpha.).multidot.