25 of the first speed change gear 3 has its front portion supported behind the input shaft 23 by the bearing 26 and its rear portion supported by the bearing 27 at the front wall 13a of the spacer transmission case 13 the first countershaft 28 has its front end supported by bearing 29 at the partition wall 11b and its rear end by bearing 30 at the front wall 13a of the spacer transmission case 13 a first counter gear train 31 on the first countershaft 28 is in mesh with a gear train 32 on the first transmission shaft 25 to form the first speed change gear 3 for the second speed change gear 5 installed in the spacer transmission case 13 the second transmission shaft 33 has its front end supported by bearing 34 at the rear end of the first transmission shaft 25 and its rear end supported by bearing 35 at the rear cover 21 the second countershaft 36 has its front end supported by bearing 37 at the rear end of the first countershaft 28 and its rear end supported by bearing 38 at the rear cover 21 the second countergear train 39 mounted on the second countershaft 36 is meshed with the gear train on the second transmission gear 33 to form the second speed change gear 5 a drive pinion shaft 41 that drives rear wheels has its front end supported immediately behind the second transmission shaft 33 by bearing 42 and its rear end supported by bearings 43 44 at the partition wall 12b of the rear transmissison case 12 a third countershaft 45 has its front end supported immediately behind the second countershaft 36 by bearing 46 and its rear end supported by bearing 47 at the partition wall 12b.