In a gas discharge tube in which a sealed envelope at least a part of which transmits light is filled with a gas, and electric discharge is generated between anode and cathode sections disposed within the sealed envelope, so as to emit predetermined light outside from the light-transmitting part of the sealed envelope, the anode section is mounted on an insulating anode support member, an insulating electrode support member having an opening for exposing the anode section is mounted on a surface surrounding the anode section, a focusing electrode having a focusing opening projecting toward the anode section is further mounted at the front face of the opening, and the cathode section is disposed on the anode support member or focusing electrode support member so as to be spaced from the focusing opening.