Unit (6) for the infusion of a substitute solution by a dialysis machine comprising a flushing device (7) internally defininga flushing chamber (9a) and comprising a connecting cone (17) extending inside the flushing chamber (9a) and suited to be hydraulically connected to a substitute solution line inside a dialysis machine； a connecting device (1) suited to connect connectingcone (17) to a duct for infusing the substitute solution to a patient； and a shutter element (8) suited to close the flushing device (7) to isolate the flushing chamber (9a) if the connecting unit (6) is in a flushing conformation. The connecting device (1) comprises a tubular element (2) having a first joining end (2a) suited to be connected to connecting cone (17) of said flushing device (7) and a second joining end (2b) suited to be connected to an infusion duct； and a blocking ring nut (3) housed with freedom of rotation around the tubular element (2) to which it is axially restrained and suited to engage and block flushing device (7) if the connecting unit (6) is in a dialysis conformation.
