A transmission shaft (62) mounting thereon fluid-operated clutch means (68) which discontinues selectively the input of power to such shaft is enclosed at one end portion thereof by a hollow cylinder (86) projected inwardly from an inner wall surface of transmission case (28) for power transmission. An annular groove (89) formed to said one end portion of shaft (62) at the outer peripheral surface thereof and communicated to the clutch means (68) via fluid path (85) in the shaft (62) is sealed by the hollow cylinder (86) so as to form an annular fluid chamber (90) which constitutes a rotary joint portion of fluid conduit line for the clutch means (68). Braking mechanism of a brake (69) associated with the clutch means (69) for braking the shaft (62) in the non-operated state of clutch is arranged within the hollow cylinder (86) by employing such cylinder as brake case. The rotary joint portion and brake (69) are thus arranged within transmission case (28) without a need of structure arranged outside the case (28).