A driven gear G3b for a forward second gear G3 which is obtained by a hydraulic clutch C3 on a second input shaft 5 is disposed contiguous with a driven gear G4b for a first gear G4. A spacer collar 9 is provided on an output shaft 6 so as to be positioned between a selector hub 8a and the driven gear G3b for the second gear G3, and the driven gear G4b for the first gear G4 is rotatably supported on the spacer collar 9.