To improve the gain of a sleeve antenna in which flowing of an unbalanced current to a radio circuit section is prevented and miniaturization is facilitated with a simple structure without use of a metallic rigid body like a balun.

In the case of feeding an antenna element 1 through a coaxial line 1, the length of the coaxial line is selected to be 1/4 wavelength with respect to the operating frequency to generate a standing wave on the antenna element 1 and an outer conductor 4 of the coaxial line so as to obtain a resonance state. Thus, the outer conductor 4 of the coaxial line acts like a radiation element, flowing of an unbalanced current to a radio circuit section 6 is prevented with a simple structure without the use of a balun, and since the coaxial line is flexible, the system is made small by using the coaxial line in a folded state and the antenna gain is improved.