To provide a housing which is capable of keeping an optical integrated circuit, such as a high-density wavelength division multiplexer(DWDM) in a narrow range of temperature, independently of a variations in the ambient temperature.

An optical integrated circuit housing 11 is equipped with a base member 12, an insulating bed 44 supported in the base member 12, a slot 46 which is provided in the insulating bed 44 to hold an optical integrated circuit, a first and a second leg 48 and 49 which are separated from each other, a heat transfer means 47 arranged in the slot 46, a heating means 54 which heats the heat insulating bed 44 mounted on one of the two legs, and a cap member 13 which is combined with the base member 12 to form a closed out housing.