An input shaft 10 is connected rotationally with a first countershaft 40 through a second main drive gear GM2V and a second main driven gear GM2N, and a second countershaft 50 is provided externally, coaxially and rotatably with respect to the first countershaft 40. A first main drive gear GM1V which is provided on the input shaft 10, always meshes with a first main driven gear GM1N, which is provided rotatably over the second countershaft 50. A fourth, sixth and seventh speed drive gear G467V, which is provided rotatably over the input shaft 10, always meshes both with a fourth speed drive gear G4V, which is provided rotatably over the second countershaft 50, and with a fourth, fifth, sixth and seventh speed driven gear G4567N, which is provided on the output shaft 60.