a glass substrate is irradiated with a converged laser beam to thereby form a spherical or nearly spherical convex portion on a surface of the glass substrate . more preferably , there may be used a method including the steps of : sticking a glass substrate to a flat member having a predetermined spherical or aspherical concave portion formed therein ; and irradiating a converged laser beam onto a surface of the glass substrate just under the concave portion while making the converged laser beam penetrate through the flat member to thereby form a spherical or aspherical convex portion in the inside of the concave portion in accordance with the shape of the concave portion .