Abstract: A mobile telephone holding device is provided for establishing a stable communication between a portable telephone and outside on-board instruments without adjusting the positions of respective infrared optical communication interfaces precisely. The holding device includes a light guide 3 disposed between a plane containing one surface of the portable telephone 5 equipped with an infrared optical communication interface 13 and one end face 19 opposing a housing 9 equipped with an IrDA element 27. The light guide 3 operates to direct an optical path of a light signal entering through one surface of the guide 3 to a direction of the other surface. The transmission and receiving of optical signals is accomplished between the telephone 5 and the instruments through the light guide 3.