Abstract: A pipe connecting structure is constituted by a milk tap having an aperture to be aligned with a corresponding opening arranged at the top of a milk delivery pipe, and a slide valve for opening and closing said aperture, and a plug-in metal fitting removably fitted to said milk tap and including an elbow pipe. The slide valve includes a slide valve carriage having a substantially circular valve sliding hole and transversal collar insertion holes disposed along the lateral sides of the valve sliding hole, a pair of guide pins projecting outwardly from the bottoms of the respective collar insertion holes, a pair of guides projecting outwardly in a direction opposite to that of the guide pins, a pair of holders for slidably holding the guides collars with tapered leading ends fitted to the respective guide pins of the valve carriage, and coil springs arranged between the collars and the bottoms of the collar insertion holes.