A hydro-static transmission travel system for a working machine enables the working machine to travel over a range from low speed to high speed with a simplified circuit arrangement. By referring to a machine traveling speed (vehicle speed) based on a detected signal from a rotation sensor, a controller computes respective target tilting amounts of hydraulic motors 10, 20 and controls the tilting amount for the hydraulic motors and a clutch. Depending on whether the traveling speed exceeds a setting speed as a reference boundary, the controller selectively switches over a drive mode between low-speed four-wheel drive, in which travel units 12, 22 are both driven, and high-speed two-wheel drive, in which the front-wheel side hydraulic motor 10 is controlled to zero displacement and only the rear-wheel side travel unit 22 is driven.