A method of deriving a path of contact of a face gear with a pinion gear includes positioning the pinion gear, shifting the pinion gear from either towards or away from the face gear along a rotational axis, deriving a first relationship between a rotational angle of the pinion gear and a fluctuation error angle, deriving second and third relationship by shifting the first relationship in either way for 360 degree divided by the number of the gear teeth, deriving a first point which the first and second relationships share and a second point which the first and third relationships share, deriving a third point, at which the fluctuation error is the same as the first and second points, between the first and second points, and determining a curve line on tooth flanks of the gear teeth on the basis of the first, second, and third points.
