To provide a portable terminal device which has an openable and closable part and an open/close detector which detects opening and closing of the part, where the portable terminal device has a reduced thickness when closed and allows easy position adjustment during assembly. A cell phone equipped with a lower part and an upper part openable and closable in relation to the lower part has a magnet and a magnetic sensor attached to the lower part; and a magnetic plate which is attached to the upper part and leads the magnetic force of the magnet attached to the lower part to the magnetic sensor attached to the lower part when the upper part is closed over the lower part.