Abstract: A housing (10) has a cavity (18) to which fluid product can be fed, and which opens into a terminal neck (20) adapted to be inserted over a bag straw. A hollow bolt (28), having a bore at its tip, is slidable in the housing between a neck-shutting position and a position in which its tip is remote from the neck mouth. A needle (34) is axially slidable in the bolt and protrudes with an enlarged head (35) within the terminal neck through the terminal bore. The needle is biased by a spring (38) to a position where the enlarged head abuts against the tip of the bolt. A drive cylinder (44) is provided for moving the bolt and the needle away from the terminal neck, for connecting the bolt to the internal cavity of the housing, for subsequently bringing the bolt near the neck in order to break the connection and to push the needle so that its enlarged head moves away from the bolt.