The discharge tube has an insulating tube formed into a hollow cylinder with one end open and the other closed. An anode electrode, which is formed as a flanged electrode, is hermetically fitted to the open end of the insulating tube and a cathode electrode, which is formed as a bar electrode, is embedded in the closed end of the insulating tube so that only the front end surface of the bar electrode faces the interior of the insulating tube. This construction allows no redundant space within the insulating tube other than the dishcarge space, so that the discharge tube can be minimized in size. This construction also permits a discharge only between the front end of the bar electrode and the inner end of the flanged electrode, stabilizing the electron emission passage or discharge path and therefore the discharge voltage.