A machine for cleaning a coupler consisting substantially of a male coupler element and a female coupler element, especially mating surfaces related parts thereof is proposed for avoiding fouling thereof in advance of practical coupling of both the elements. The machine is provided with a ring tube type cleaner unit suspended from a detachable top cover of a cleaning box. The inside space of the ring tube constitutes a liquid chamber supplied with cleaning water from a source and delivers from a number of nozzle openings formed through the inner wall of the ring tube, for delivery of sweep water jets in the center of the ring tube, where the tip and to-be-coupled ends of both the coupler elements are positioned at a small mutual distance. After cleaning jobs, either one of the male and female coupler elements is advanced to mating position for establishing a tight contact with the other one of the coupler elements for avoiding otherwise frequently encountered, disadvantageous fouling.