A steering transaxle comprises: left and right drive shafts drivingly connected to respective steerable wheels; a differential gear unit differentially connecting the drive shafts to each other; a transaxle casing having an opening, the transaxle casing incorporating the differential gear unit and the drive shafts; a cover for covering the opening of the transaxle casing; a variable hydraulic motor having a movable swash plate, the hydraulic motor being provided on the inside of the cover; a motor control mechanism for controlling the movable swash plate, the motor control mechanism being provided on the outside of the cover; and a hydraulic oil port for oil supply and delivery to and from the hydraulic motor, the hydraulic oil port being provided on the outside of the cover. The cover, the hydraulic motor with the movable swash plate, the motor control mechanism and the hydraulic oil port constitute an assembly unit, which is detachably attached to the transaxle casing so that, by covering the opening with the cover, the hydraulic motor is disposed in the transaxle casing so as to be drivingly connected to the differential gear unit.