To obtain a built-in antenna having good radiation characteristics adapted to vertical polarization regardless of the direction of a cellular phone, the antenna being compact, low-cost, and low power consumption.

A parasitic element 130 (resonant element) of electric length λ/2 is provided in parallel to a driven element 120 of electrical length approximately λ/4. Then, the parasitic element 130 is resonated by feeding the driven element 120 so as to radiate a polarized component of the longitudinal direction of the central part 131 of the parasitic element 130. Further, the parasitic element 130 and a substrate ground 110 (grounded conductor) are short-circuited by switching an SPDT switch 140 (switch) so that the parasitic element 130 will not resonate. Therefore, unbalanced current flows the substrate ground 110 so as to radiate the polarized component in the longitudinal direction of the substrate ground 110. Accordingly, optimal irradiation characteristic can be selected by operating the SPDT switching 140 in accordance with the direction of a mobile phone (portable communication device) equipped with an antenna 100.