The stem slide device of the present invention in an extruder has a stem vertical movement support (71) and slide guide members (72-1, 2) fastened to the support and forming guide grooves. A stem (6) pushing against a billet loaded in a container is held at the slide table (73) horizontally. The slide table slides in the vertical direction along the vertical face of the support. When the slide table is positioned at the bottom end of the guide groove, the hydraulic cylinders (77-1, 2) are driven so that the rods (78-1, 2) push the back surface of the slide table against the vertical face of the support. Therefore, the axis of the stem is held matched with the axis of the container.