According to one embodiment an extractor is provided that comprises a stem with a handle at one end and a conical spacer engagement device cooperating with the conical spacer for extracting it from the concrete wall at the other end. The extractor also comprises a mobile element which is displaceable by a user along the stem between a retracted position to which the mobile element is moved in order to contribute to the extraction of the conical spacer, and an advanced position to which the mobile element is moved in order to push the conical spacer, releasing it from the conical spacer engagement device. When the conical spacer engagement device is fixed to the conical spacer, then the mobile element is moved to the retracted position, the corresponding conical spacer being extracted. The mobile element is then moved again to the advanced position, releasing the conical spacer from the conical spacer engagement device.