To reduce the outermost diameter of a spiral antenna without changing the lower working frequency limit of the antenna by connecting a radio wave reflection suppressing member to the end point of a conductor line.

Extension lines 1e and 1f made of metallic conductor are connected to end points 1c and 1d, i.e., winding end parts of spiral conductors 1a and 1b at each of both ends of lines 1e and 1f, and then extend toward a bottom plate in parallel to the points 1c and 1d respectively. Thus, both lines 1e and 1f pierce through an absorber 5 that is filled into a case 3 of a cavity structure. Thus, waves reflected from the points 1c and 1d are absorbed by the absorber 5 for the lines 1e and 1f which are connected to the end points of the lines 1a and 1b. Thus a radiation pattern is not disturbed even with frequencies radiated from the points near the winding end parts of the lines 1a and 1b. As a result, a satisfactory antenna radiation pattern is secured even with a lower frequency limit.