In a disc molding die, a stationary base plate (15) is attached to a stationary platen, and a stationary mirror-surface plate (16) is attached to the stationary base plate (15). Further, a movable base plate (35) is attached to a movable platen, and a movable mirror-surface plate (36) is attached to the movable base plate (35). A stationary guide ring (18) is attached to the stationary base plate (15) so as to surround the stationary mirror-surface plate (16), while a movable guide ring (38) is attached to the movable base plate so as to surround the movable mirror-surface plate (36). When the die is clamped, the movable guide ring (38) is brought into contact with the stationary guide ring (18). A cavity ring (71) is detachably attached to one of the stationary and movable mirror-surface plates (16,36) such that the cavity ring (71) projects toward the other of the stationary and movable mirror-surface plates (16,36). An abutment position adjusting mechanism adjusts the abutment position where the movable guide ring (38) abuts on the stationary guide ring (18), in accordance with the thickness of the cavity ring (71). This makes it possible to change the thickness of the cavity (C) without replacing the mirror-surface plates (16,36).