Abstract: An automatic opening device for a carton package comprising a spout (3) and a cap (26), the device being provided with a plurality of protruding cutting elements (30) onto the internal face (29) of a cap element (27) thereof. The spout (3) is provided with a thread (2) having grooves (2c) and (2f) of opposite orientation and connected each other, within which pass internal screw thread segments (19) distributed equidistantly in the internal part of the elongate lateral wall (28) of the cap (26). The spout is applied in a carton package over a hole in the carton layer which exposes the aluminium layer. When the cap (26) is unscrewed from the spout (3) the first time it makes a downward axial movement and next the protruding cutting elements (30) are able to cut the aluminium layer of the carton package in order to open a passage for the content of the carton package.