A discharge tube 10 formed by forming an airtight envelope 16 by hermetically sealing openings at both ends of a case member 12 made of an insulating material opened at both ends with a pair of cap members 14, 14 that double a discharge electrode, forming a predetermined discharge gap 22 between discharge electrode portions 18, 18 of the cap members 14, 14, forming on an inner wall surface 24 of the case members 12 a plurality of linear triggering discharge films 28 of which both ends are disposed separated by a small discharge gap 26 opposite to the cap members 14, 14 that double a discharge electrode, forming on a surface of the discharge electrode portion 18 a film 30 containing an alkali iodide, and encapsulating a discharge gas containing Kr (krypton) in the airtight envelope 16.