With respect to a highly flat glass bulb for cathode ray tube having an effective screen diameter along a diagonal axis of a glass panel 10 of more than or equal to 500 mm, and an average radius of curvature of an outer surface of a face portion of more than or equal to 10,000 mm, a ratio between a length h of a skirt portion 13 of the glass panel and a distance H in the tube axis direction from an edge surface portion where a funnel is sealed with the glass panel to a reference line is defined so as to fall within a predetermined range in relation to a deflection angle of the funnel representing substantial degree of divergence in the tube axis direction of the funnel, thereby achieving reduction of weight of the glass bulb while maintaining a certain mechanical strength, as well as suppressing breakage during frit seal heating process.