A mold for forming an optical disk base provided with a movable specular plate, a stationary specular plate, and an annular stamper having inner and outer peripheral edges. The reverse surface of the stamper is in contact with one of the specular plates, and the stamper is supported at its inner end portion by an inner stamper retainer and at its outer end portion by an outer stamper retainer. A cavity is formed between the stamper and the other one of the specular plates when the mold is closed. This mold further has: a circumference ring having an extreme end surface located inside the outer stamper retainer in the radial direction and facing a surface of the stamper, and an inner peripheral surface defining an outer peripheral surface of the cavity, the circumference ring being supported on the other specular plate so as to be movable in the mold opening/closing direction; and an actuator for moving the circumference ring between a position at which the extreme end surface abuts against the stamper and a position at which the extreme end surface is flush with the specular surface of the other specular plate or is recessed from this surface away from the cavity.