A hydraulically operated power transmission apparatus transmits the output power from a power source at a selected speed ratio. The power transmission apparatus includes a casing having an opening in one end thereof, a base plate attached to the end of the casing in covering relation to the opening, the casing and the base plate jointly serving as a transmission case defining a closed space therein, a hydraulic pump having a pump drive shaft adapted to be operatively coupled to the power source, the hydraulic pump being disposed in the closed space, a hydraulic motor having a motor output shaft and drivable by a hydraulic pressure generated by the hydraulic pump, the hydraulic motor being disposed in the closed space, a pair of output shafts extending through the casing and the base plate, respectively, and disposed coaxially with each other, a differential mechanism disposed in the closed space and operable by the motor output shaft, the differential mechanism being coupled to the output shafts. The hydraulic pump, the hydraulic motor, and the differential mechanism are mounted on the base plate. The pump drive shaft, the motor output shaft, and the differential mechanism have ends rotatably supported by the base plate and opposite ends rotatably supported by the casing.