A hydraulically operated continuously variable transmission comprises a hydraulic pump and a hydraulic motor having a motor cylinder including an integral tubular member. A distribution disc is coupled to the motor cylinder at one end of the tubular portion and has oil passages communicating with the hydraulic pump and the hydraulic motor. A hollow fixed shaft is inserted in the tubular member, and a distribution ring is axially movably mounted on the fixed shaft and has an end face slidably held against the distribution disc. A mechanical seal assembly includes an annular rotatable seal member rotatably disposed in the tubular member, and an annular fixed seal member axially movably, but nonrotatably, mounted on the fixed shaft. A compression spring is disposed between the fixed seal member and the distribution ring and is resiliently extensible for applying a preload to abut an axial end face of the fixed seal member against an axial end face of the rotatable seal member and a preload to abut the end face of the distribution ring against the distribution disc.