A gas discharge tube 10 of the present invention generates an electric discharge between an anode part 24 and a cathode part 52 disposed inside a sealed container 12 in which gas has been sealed. The gas discharge tube 10 includes 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 46 for narrowing an electric discharge path between the anode part and the cathode part; and a supporting part 30 which supports the electric discharge path restricting part and which is electric insulating. The electric discharge path restricting part is provided at its tip portion with a projecting part 44 which is cylindrical and projects toward the cathode part side. Letting the outer diameter and height of the projecting part 44 be “D” and “H,” respectively, then D/H is in the range of 0.5 to 2.0. With this structure, it is possible to generate an intense electric field in the vicinity of the tip of the projecting part. This can decrease a startup voltage, thereby ensuring the generation of an electric discharge.