The invention relates to a plastic screw plug (6) for attaching a rail to a sleeper, in particular concrete sleeper, having an essentially cylindrical plug body which has at the head end a stem region (7) which preferably adjoins a plug crown (8) toward the bottom, and in the continuation of which stem region (7) has an outer thread (9) with which it can be turned out of or into the sleeper, and is provided with an inner thread (10) underneath the stem region (7) in order to receive a sleeper screw. The intention is to improve such a plastic screw plug, in particular, with regard to its operating properties such as uniform application of force into the sleeper, simpler mounting and replaceability, reduction of the risk of fracturing, and additionally to permit it to be used in a variable way. For this purpose, the stem region (7) has on the inside, in the junction with the inner thread (10), a shoulder (13) which reduces the internal diameter over part of the length of the stem region (7) and extends in a spiral shape over at least part of the circumference.