PROBLEM TO BE SOLVED: To provide a transmission which can use one non-circular gear pair for both up-shift and down-shift.SOLUTION: Between an input member and an output member, at least three first-third stage gear pairs and the non-circular gear pair are selectively connected via clutches. The non-circular gear pair includes constant-speed engagement sections 26a-26d; 36a-36d in which it is the same in reduction ratio as the gear pairs, and variable-speed engagement sections 26s-26v; 36s-36v in which the reduction ratio gradually increases/decreases between the adjacent constant-speed engagement sections 26a-26d; 36a-36d. The non-circular gear pair also includes intermediate stage constant-speed engagement sections 26b, 26d; 36b, 36d set in regions where the reduction ratio decreases from a maximum reduction ratio stage to a minimum reduction ratio stage and the reduction ratio increases from the minimum reduction ratio stage to the maximum reduction ratio stage, respectively, when the engagement process of the non-circular gear pair ends.