Abstract: Amplitude of transmission error of tooth pair of a gear is obtained in relation to actual contact ratio (.epsilon..sub.r) which is an angular range (.theta..sub.r) of actual contact of the surfaces of the tooth pair divided by an angular pitch (.theta..sub.p) of the gear. The specifications and/or tooth surface modification of the gear is/are are determined so as to reduce the transmission error amplitude under non-load condition in which the actual contact ratio (.epsilon..sub.r) is 1.0. Alternatively, the specifications and/or tooth surface modifications is/are determined so as to increase an effective contact ratio (.epsilon..sub.n) which is an angular range (.theta..sub.n) divided by the angular pitch (.theta..sub.p), the angular range (.theta..sub.n) corresponding to a portion of a path of the tooth contact point in which an edge contact of the tooth pair does not take place at the tooth top or at the ends of the face width of the teeth.