A transmission and a gear changing method capable of changing a reduction ratio while a load is supported without stopping rotation, and transmitting a rotation angle accurately and power efficiently.First and second gear element pairs (16, 17) are arranged between an input member (12) and an output member (14) via first and second clutches (40, 42), and a non-circular gear pair (18) is arranged via a non-circular gear element pair-use clutch (44). The non-circular gear pair (18) includes between the input member (12) and the output member (14) a first meshing section where it has a ratio equal to a first reduction ratio when the first gear element pair (16) is coupled, and a second meshing section where it has a ratio equal to a second reduction ratio when the second gear element pair (17) is coupled. When the gear element pairs (16, 17) to be coupled between the input member (12) and the output member (14) are switched, the non-circular gear element pair (18) is coupled between the input member (12) and the output member (14) transitionally.