PURPOSE: To enable injection molding of a high density information recording and storing plastic disc which has the uniform wall thickness distribution in the radial direction and also has excellent configurational stability by enlarging the axially directional clearance dimension of an annular molding cavity successively as being separated from the injection gate to the radial direction.

CONSTITUTION: The axially directional clearance dimension of an annular molding cavity 3 is enlarged successively as being separated from the injection gate 5 to the radial direction. Namely, the cavity formed by being defined by split molds 1, 2 are the same to the conventional ones; however, the axial wall thickness of a stamper 4 is tapered so that it becomes thin as separated from the gate 5; in other words, it is separated from the virtually central surface 6 of the molding cavity. The dimension of the cavity formed by being defined in this mold assembly is thin in its central part and thick in its peripheral part Thereby a high density information recording and storing plastic disc can be injection-molded which has the uniform wall thickness distribution in the radial direction and also has excellent configurational stability.