A continuously variable hydraulic transmission operatively interconnects a transmission mechanism and a clutch mechanism. The hydraulic transmission includes an oil pump having an inlet port and an outlet port and a transmission ratio control member such as a swash plate which is displaceable for varying the transmission ratio between a higher-speed position, a lower-speed position, and a neutral position, an oil motor having an inlet port and an outlet port and drivable by the oil pump, a first oil passage hydraulically interconnecting the outlet port of the oil pump and the inlet port of the oil motor, a second oil passage hydraulically interconnecting the inlet port of the oil pump and the outlet port of the oil motor, a housing accommodating the oil pump, the oil motor, the first oil passage, and the second oil passage, and an oil passage control unit for selectively bringing the first oil passage into and out of hydraulic communication with the second oil passage. The oil passage control unit is fully accommodated in the housing and actuatable by the transmission ratio control member to bring the first and second oil passages into hydraulic communication with each other only when the transmission ratio control member is near the neutral position.