A dual-clutch transmission, comprising: a clutch device having first and second clutches (21, 22) capable of connecting or disconnecting power from a driving source; a first input shaft connected to the first clutch; a second input shaft connected to the second clutch; a subsidiary shaft arranged parallel to the first and second input shafts (31, 32); an output shaft arranged parallel to the subsidiary shaft; a first primary gear train that transmits power from the first input shaft to the subsidiary shaft; a secondary primary gear train that transmits power from the second input shaft to the subsidiary shaft; a plurality of output gear trains (51, 52, 53) rotatably provided on the subsidiary shaft and the output shaft; and a synchronization mechanism for synchronously joining the output gear trains (51, 52, 53); wherein a first gear-shift stage is set when the first clutch is in a connecting state and the second clutch is in a disconnecting state by the clutch device in a state where one of the output gear trains is synchronously joined by the synchronization mechanism, wherein an upshifting to a second gear-shift stage, which is one stage higher than the first gear-shift stage, is performed when the first clutch is changed to a disconnecting state and the second clutch is changed to a connecting state by the clutch device in a state where the one of the output gear trains is synchronously joined by the synchronization mechanism, wherein an upshifting to a third gear-shift stage, which is one stage higher than the second gear-shift stage, is performed when the first clutch is changed to the connecting state and the second clutch is changed to the disconnecting state by the clutch device in a state where another output gear train different from the one of the output gear trains is synchronously joined by the synchronization mechanism, and wherein a step ratio upon gear shift from the second gear-shift stage to the third gear-shift stage is set to be smaller than a step ratio upon gear shift from the first gear-shift stage to the second gear-shift stage. a clutch device having first and second clutches (21, 22) capable of connecting or disconnecting power from a driving source; a first input shaft connected to the first clutch; a second input shaft connected to the second clutch; a subsidiary shaft arranged parallel to the first and second input shafts (31, 32); an output shaft arranged parallel to the subsidiary shaft; a first primary gear train that transmits power from the first input shaft to the subsidiary shaft; a secondary primary gear train that transmits power from the second input shaft to the subsidiary shaft; a plurality of output gear trains (51, 52, 53) rotatably provided on the subsidiary shaft and the output shaft; and a synchronization mechanism for synchronously joining the output gear trains (51, 52, 53); wherein a first gear-shift stage is set when the first clutch is in a connecting state and the second clutch is in a disconnecting state by the clutch device in a state where one of the output gear trains is synchronously joined by the synchronization mechanism, wherein an upshifting to a second gear-shift stage, which is one stage higher than the first gear-shift stage, is performed when the first clutch is changed to a disconnecting state and the second clutch is changed to a connecting state by the clutch device in a state where the one of the output gear trains is synchronously joined by the synchronization mechanism, wherein an upshifting to a third gear-shift stage, which is one stage higher than the second gear-shift stage, is performed when the first clutch is changed to the connecting state and the second clutch is changed to the disconnecting state by the clutch device in a state where another output gear train different from the one of the output gear trains is synchronously joined by the synchronization mechanism, and wherein a step ratio upon gear shift from the second gear-shift stage to the third gear-shift stage is set to be smaller than a step ratio upon gear shift from the first gear-shift stage to the second gear-shift stage.