Abstract: There is disclosed a coaxial lightning arresting structure comprising an inner conductor, an outer conductor surrounding the inner conductor and an arrester tube located between the inner and outer conductors perpendicularly to the direction of transmission and connected at its ends to the inner and outer conductors, respectively. The portion of the inner conductor provided with the arrester tube has a reduced effective sectional area as compared with the other portions of the inner conductor so that the ratio of the outer conductor inner diameter to the outer diameter of the inner conductor portion having the reduced effective sectional area is greater than the ratio of the outer conductor inner diameter to the inner conductor outer diameter at the other portions of the inner conductor which latter ratio provides the coaxial transmission line with a given characteristic impedance.