An irrigation system comprises a water supply line located on the ground having a plurality of equally spaced valve assemblies and an irrigation machine movable along the water supply line. The irrigation machine comprises a base frame situated on the ground to move along the water supply line, propelling means provided on the base frame for moving the base frame along the water supply line at a constant speed, water transfer means on the base frame for supplying water onto the ground, and a coupling assembly on the base frame. The coupling assembly includes at least two coupling sections connected to each other to freely move relative to the base frame so that when one of the coupling sections is moved in one direction along the longitudinal direction of the base frame, the other coupling section is moved in the opposite direction along the longitudinal direction of the base frame. The coupling sections are connected to the water transfer means at respective ends and are engaged at the other end with one of the valve assemblies of the water supply line alternately. At least one of the coupling sections is always connected to one of the valve assemblies.