PURPOSE: To generate large infrared-ray flux with all wavelengths and to improve energy efficiency by providing a radiation cavity in a black material light source as a small-sized, easy-to-replace light source element which is used for an infrared spectrophoto-meter, etc.

CONSTITUTION: The light source element 15 is formed of a cylindrical ceramic rod 2 which has a furnace winding 1 provided lengthwise radially around the outer circumference. Heat from an electric wire is distributed uniformly and the winding should not short-circuit. For this purpose, the furnace winding 1 is set in grooves 3 formed at equal intervals in the outer circumference of the ceramic rod 2. The radiation cavity 4 is formed at one terminal of the ceramic rod 2 along its center axis and concentrically with the winding 1. The radiation cavity 4 is preferably a blind hole by drilling. The furnace winding 1 is connected to a power source in operation and heated. Consequently, the ceramic rod 2 is heated. The length of the cavity 4 is within a length range wherein the ceramic rod 2 is heated uniformly in the lengthwise direction of the cavity 4.