PURPOSE: To make principal parts almost possible to be used in common with a specification change between n+1-speed direct-coupling type and n-speed direct-coupling type as well as to aim at cost reduction by making a direct- coupling selector synchromesh mechanism and an n-speed/n-speed selector synchromesh mechanism solidly operable.

CONSTITUTION: When a 5-speed direct-coupling type transmission 4 is changed to a 4-speed direct-coupling type in specifications, a fourth gear 11 is disused, while a 3-4 speed selector synchromesh mechanism 12 and a direct-coupling selector synchromesh mechanism 14 are made so as to be interlocked with each other by an integral shift fork 20. Since the 5-speed direct-coupling type can be changed to the 4-speed direct-coupling type in specifications by only removal of the fourth gear 11, a specification change is very easily performable, while other principal parts (an input shaft 4, an output shaft 5, a countershaft 6, casings 22, 23, 24 or the like) can all be commonly used with the 5-speed direct-coupling type so that cost reduction is realized.