There are provided: an HST hydraulic circuit that is formed through a closed circuit connection of a variable displacement hydraulic pump 2 and first and second variable displacement hydraulic motors 3 and 4; a charge pump 6 that replenishes the HST hydraulic circuit with hydraulic oil; a cooling circuit through which an excess of the hydraulic oil that has been replenished from the charge pump 6 to the HST hydraulic circuit is returned to a reservoir through the hydraulic pump 2, the first hydraulic motor 3, and the second hydraulic motor 4; a hydraulic source 7 that supplies hydraulic oil to an hydraulic device other than the first and second hydraulic motors; and a merge circuit through which a portion of hydraulic oil from the hydraulic source 7 is merged into an intermediate oil line 22 that lies downstream of the first hydraulic motor 3 and upstream of the second hydraulic motor 4 in the cooling circuit.