PURPOSE: To provide a transmission line antenna device which can transmit/ receive a radio wave with high gain even when the radio wave is vibrated in any direction inside a plane orthogonal to a main transmission/reception direction concerning the relevant direction.

CONSTITUTION: Concerning a transmission line antenna system 2 on a metallic casing 1, the length from a feeding terminal 2a to a feeding terminal 2o and the length from the feeding terminal 2a to a ground terminal 2t are made the 1/2 of a resonant wavelength, the length near the equally bisecting point of parallel elements 21 and 2q is made the 1/4 of the resonant wavelength so that a vertical antenna can look like only one antenna composed of a vertical element 2b, and parallel elements 2j, 2m and 2r and these mirror images are integrated so as to look like one horizontal antenna.