PURPOSE: To reduce the antenna efficiency and to avoid the production of loss due to mismatching with a circuit by selecting the coil such that the cross sectional area of the coil, the pitch and the total length are all selected within 1/10 of a wavelength and the cross sectional area and the pitch of the coil satisfy a specific relation to the wavelength.

CONSTITUTION: The cross sectional area S, the pitch (p) and the total length 2Np of the coil are selected all to be 1/10 of a wavelength λ or below and the pitch (p) of the across sectional area of the coil satisfy the relation of 2πpλ with respect to the wavelength. As a result, the characteristic of making the input impedance unchanged is obtained depending on the presence or absence of an electric image, that is, the presence or absence of a metallic plate in proximity with the coil. Thus, the deterioration in the antenna efficiency due to the production of mismatching loss with the circuit or deterioration in the radiation resistance is avoided.