PURPOSE: To mold a sub-substrate fat supporting a magneto-optical recording layer by horizontally mounting a metal mold and allowing a hot liquid plastic material to flow uniformly in a radial direction from the symmetrical axis of the plane of this metal mold.

CONSTITUTION: A polycarbonate material 28 is packed into the central part of the metal mold where a perpendicular axis 29 intersects with the horizontal plane 30 regulated by the cavity 32 of the metal mold 19. The material formed to a liquid form by heating is made to flow symmetrically in the radial direction within the cavity. Since there are no gravity components acting on undesirable directions, the substrate 20 having the stable amplitude of read signals is obtd. The thickness of the cavity suffices with about 1.2mm. The one surface 32 of the metal mold 19 is provided with a grooved stamper 33 to form the grooves 34 of the substrate 20, whose grooves are used for tracking. The parts 41 of the metal mold 19 are fixed and the parts 42 are separable. The plastic material is supplied from a hopper 35 to a heating and mixing system where the plastic material is advanced by a member 37 and the hot liquid material 28 is supplied downward to the spool 40 of the metal mold 19. Molding by compression molding is otherwise equally well.