of a disc type having a brake drum 25a carried by a support 25b which is connected with the planetary carrier 14a of the gear unit 14 a disc carrier 25c is mounted through axial splines on the casing 11 the brake drum 25a and the disc carrier 25c on the casing 11 of the transmission carry alternately arranged brake discs 25d in the casing 11 there is formed an annular cylinder 25d in which a piston 25e is disposed the piston 25e defines a pressure chamber 25g in the cylinder 25d so that hydraulic pressure applied to the pressure chamber 25g forces the piston 25e toward the brake discs 25c to thereby make the clutch discs 25c engage with each other a return spring 25f is provided to force the piston 25e in the brake releasing direction the one way clutch 26 is provided adjacent the brake 25 the one way clutch 26 includes an outer race 26 a which is mounted on the casing 11 through axial splines an inner race 26b connected with the support 25b between the outer and inner races 26a and 26b there are sprags 26c the gear mechanism 10 provides four forward drive gear stages and one reverse drive gear stage which can be selected by appropriately engaging the clutches 20 21 24 and 27 and the brakes 23 and 25 the relationship between the gear stages and the operations of the clutches and brakes are shown in.