A compression apparatus which is operated in association with at least two mold parts which can move relative to one another. The compression apparatus is formed of at least one compression units having a working chamber and a working element which can move in it. A gaseous medium can be provided to the working chamber under any initial pressure. During the closing motion of the parts of the mold, the working element is moved in the working chamber, such that the volume in the working chamber is reduced, and in this way, in the course of the closing motion, the gaseous medium is compressed in the working chamber. The compressed gaseous medium can then be sent to a consumer with or without intermediate storage. A compression apparatus of this type is intended for an injection molding machine in which gas pressurization is to take place in a cavity. In one embodiment, the compression apparatus integrated into the mold or into one of two mold parts, or its platens, adapter plates or the like, and in another form, it is attached to one of the mold parts. Optionally, the compression apparatus can comprise several compression stages formed by a series connection of a plurality of individual working chambers, with or without cooling of the gaseous medium as it moves between the compression stages.