A preform for an optical element is provided which involves less amount of deformation of glass in molding, and readily improves lifetime of the mold. The present preform for molding an optical glass element 10 exhibits an almost circular shape having a predetermined diameter in top view, exhibits a flattened semicircular shape having a predetermined overall height in side view, has a concave face on the top surface such that the predetermined overall height is attained at approximately the central position of the aforementioned circular shape, and has a concave face so as to fit along the convex face of the top surface such that a space is provided on the bottom face. The under surface may be either a concave face or a convex face. Also, the top surface may be either a concave face or a convex face.