The invention relates to an internal multiband antenna intended for small-sized radio devices, and a radio device with such an antenna. The basic structure of the antenna is a two-band PIFA. A parasitic element (230) is added to it inside the outline of the radiating plane (220) of the PIFA, e.g. in the space (229) between the conductor branches (221, 222) of the radiating plane. The parasitic element extends close to the feeding point (FP) of the antenna, from which place it is connected to the ground plane of the antenna with its own short-circuit conductor (235). The structure is dimensioned so that the resonance frequency based on the parasitic element comes close to the one resonance frequency of the PIFA, thus widening the corresponding operating band, or a separate third operating band is formed for the antenna with the parasitic element. Because the parasitic element is located in the central area of the radiating plane and not in its peripheral area, the radio device user's hand does not significantly impair the matching of the antenna on an operating band which has been formed by the parasitic element. In addition, when the resonance frequency based on the parasitic element is on the upper operating band, the matching of the antenna also improves on the lower operating band.