A twin clutch type speed-change apparatus comprising a transmission having six gear pairs disposed between a main shaft (6, 7) and a countershaft, and a twin clutch provided on the main shaft (6, 7), a rotational drive force from a power source being connected/disconnected between the transmission and the power source by the twin clutch, wherein the main shaft (6, 7) is composed of an inner tube, also denoted by inner main shaft, carrying three odd speed-change step gears (M1, M3, M5), also denoted by first-, third- and fifth-speed drive gears, and an outer tube, also denoted by outer main shaft, carrying three even speed-change step gears (M2, M4, M6), also denoted by second-, fourth- and sixth-speed drive gears; wherein the twin clutch includes a first clutch adapted to connect/ disconnect a rotational drive force transmitted to the inner tube and a second clutch adapted to connect/disconnect a rotational drive force to the outer tube; wherein the countershaft carries six driven gears (C1, C2, C3, C4, C5, C6), also denoted by first-, second-, third-, fourth-, fifth- and sixth-speed driven gears meshed with the six drive gears (M1, M2, M3, M4, M5, M6) respectively, wherein the first-speed through fourth-speed driven gears (C1, C2, C3, C4) are attached to the countershaft so as to be incapable of axial slide and circumferentially rotatable, wherein the fifth-speed and sixth-speed driven gears (C5, C6) are attached to the countershaft so as to be axially slidable and incapable of circumferential rotation, wherein the first-speed drive gear is formed integrally with the inner main shaft, wherein the third-speed drive gear is attached to the inner main shaft so as to be axially slidable and incapable of circumferential rotation, wherein the fifth-speed drive gear is attached to the inner main shaft so as to be incapable of axial slide and circumferentially rotatable, wherein the second-speed drive gear is formed integrally with the outer main shaft, wherein the fourth-speed drive gear is attached to the outer main shaft so as to be axially slidable and incapable of circumferential rotation, wherein the sixth-speed drive gear is attached to the outer main shaft so as to be incapable of axial slide and circumferentially rotatable, wherein each slidable gear (M3, M4, C5, C6) is slid by a corresponding one of shift forks (71, 72, 81,82) each engaging a corresponding one of the slidable gears to connect/ disconnect the rotational drive force between the slidable gear (M3, M4, C5, C6) and the nonslidable gear (M5, M6, C1, C2, C3, C4) coaxially adjacent thereto, wherein the first-speed and second-speed drive gears do not represent non-slidable gears; wherein a dog clutch mechanism is applied to a structure of connecting or disconnecting the rotational force between the slidable gear and the non-slidable gear coaxially so as to transmit the rotational drive force by engaging a dog tooth with a dog hole, wherein the fifth-speed driven gear is formed with dog teeth which are configured such that two types of dog teeth (55, 56) different in axial height from each other are alternately arranged, the dog teeth (55, 56) being circumferentially spaced apart from each other at regular intervals, and wherein the configuration of the dog teeth being in different axial height and engage with the single dog hole is only applied to the dog clutch between the first-speed driven gear and the fifth-speed driven gear.