A gas discharge tube 10 of the present invention generates an electric
discharge between an anode part 24 and a cathode part 56 disposed inside
a sealed container 12 in which gas has been contained. The gas discharge
tube 10 comprises an electric discharge path restricting part 28 which is
cylindrical and disposed between the anode part and the cathode part, and
which has a throughhole 42 for narrowing an electric discharge path
between the anode part and the cathode part; and an electric discharge
shielding part 50 which is disposed to surround the electric discharge
path restricting part so as to restrict an electric discharge from the
outer peripheral surface of the electric discharge path restricting part
to the cathode part, and which is electrically isolated from the electric
discharge path restricting part. The end part of the electric discharge
path restricting part on the cathode part side projects beyond a surface
of the electric discharge shielding part on the cathode side by a
predetermined amount "P" of projection. With this structure, a high
density electron region is formed exclusively in a part of the
throughhole of the electric discharge path restricting part on the
cathode side, thereby ensuring the generation of a startup discharge.