A portable terminal device is disclosed that includes a movable part having a display part; an arm part supporting the movable part so that the movable part is rotatable at least ±90° with reference to a rotation center position around a rotation center; and a position detection part detecting the position of the movable part relative to the arm part. The position detection part includes magnets, magnetic sensors detecting the magnetic fields thereof, and a calculation part determining the position of the movable part based on sensor outputs. The magnets are provided in one of the arm part and the movable part so as to be 90° apart from each other around the rotation center. The magnetic sensors are provided in the other one of the arm part and the movable part so as to oppose the corresponding magnets when the movable part is positioned at the rotation center position.