An apparatus for plasticizing thermoplastic synthetic plastics material includes a screw (13) which rotates within a housing (12) and plasticizes the synthetic plastics material which is supplied by a supply mechanism (27) through an intake opening (10) of the housing (12) of the screw (13). The housing (12) of the screw (13) is provided on its inner wall with at least two wide pockets (23,24). These pockets (23,24) extend in longitudinal direction of the screw (13) and are separated from each other by ribs (25,26) which are narrow, when compared with the width of the pockets. One pocket (24) is open towards the intake opening (10), the other pockets (23) are closed by at least one rib (25,26) of the housing (12) of the screw (13) with respect to the intake opening (10). In order to improve filling of the screw (13), the supply mechanism (27) for the synthetic plastics material to be treated cooperates with the screw (13). This supply mechanism (27) stuffs the synthetic plastics material into the intake opening (10) of the housing (12) of the screw (13) with a component of motion which is directed perpendicularly to the longitudinal axis of the screw (13). When seen in direction of rotation of the screw (13), the depth of the pocket (24) which is open with respect to the intake opening (10), is at least constant. Alternatively thereto or in combination therewith, at least one rib (25,26) may be adjustable in order to enable one to change the take-up behavior of the screw (13) with respect to the synthetic plastics material to be treated.