A transmission is provided in which gear trains for multiple shift stages are provided between a countershaft and a first main shaft and second main shaft running coaxially and relatively rotatably through the first main shaft so that the gear trains can be alternatively established, wherein a plurality of odd number stage gear trains (G1, G3, G5) are provided between the second main shaft (27) and the countershaft (28) so that the first speed gear train (G1) is disposed on the opposite side to the first and second clutches (37, 38), an end part of the second main shaft (27) on the first speed gear train (G1) side is rotatably supported on an engine main body via a main shaft side ball bearing (31) having a larger diameter than an outer diameter of a first speed drive gear (89) provided integrally with the second main shaft (27), and an outer race (31a) of the main shaft side ball bearing (31) is fixed to the engine main body via a fixing plate (95) engaging with an outer peripheral part of the outer race (31a). This enables the transmission to be made compact in a direction along the axis of the first and second main shafts.