A gas discharge tube comprises a hermetically sealed container encapsulating gas, a cathode portion and an anode portion (12) arranged in the hermetically sealed container in order to generate discharge, and a discharge path limiting portion (26) arranged between the cathode portion and the anode portion (12) in order to constrict the discharge path. The anode portion (12) is provided with an opening (12a) having a noncircular shape. More specifically, the opening has a maximum opening width in the vertical direction larger than that in the horizontal direction. With such a structure, the quantity of visible light radiated from a visible light source arranged on the back of the gas discharge tube, passing through the opening (12a) and a hole (26a) made in the discharge path limiting portion (26) and being irradiated forward together with the light radiated from the gas discharge tube can be increased while deterioration in discharge starting performance or shortening in lifetime of the anode portion being pre
