In the gas
discharge tube according to the present invention, there is carried out narrowing of the
discharge path with cooperation of the first opening 20 and the second opening 12 in order to obtain higher luminance of light. Further, in order to maintain excellent starting-properties of a lamp even if the
discharge path is narrowed, a predetermined
voltage is applied to a second discharge path limit portion 11 externally. Thereby, a positive or active starting discharge is produced in such a manner as to pass through the first opening 20. Further, the second opening 12 is comprised of not only a straight section 13 extending in a direction of an
optical axis Y, but also a spread section 14 extending from an end portion of the straight section 13 toward the first opening 20. The spread section 14 has a function of improving the starting properties of the lamp and forms an arc ball, and the straight section has a function of improving a
plasma density. Thereby, discharge at a starting time is made easy to pass through the second discharge path limit portion 11. As a result, a rapid start of discharge between a
cathode 23 and an
anode portion 8 is achieved in the second discharge path limit portion 11, and that contributes to proper generation of an arc ball after lighting.