A device for matching an antenna impedance in a portable radio telephone having transmission and-receiving circuits, a foldable casing enclosing the radio, the foldable casing movable between an unfolded position and a folded position, an antenna movable between an extracted position from the foldable casing and a retracted position into the foldable casing includes means for sensing whether the foldable casing is in the unfolded position and for sensing whether the antenna is in the extracted position, and for providing a sensing signal in response thereto, and means for matching an impedance of the antenna and an impedance of the radio in response to the sensing signal, thereby making an optimal matching of impedances between the antenna and the radio according to the states of the folder casings and the antenna, and according to the transmission / reception mode.