The invention is directed to a process for manufacturing a protective cap which is adapted to be fitted to an ear canal temperature measurement probe of an infrared radiation thermometer and to be introduced into a body cavity. The protective cap (1) is formed of a base body (3) of plastic material having its one end open and its opposite end closed by a window film (7) transparent to infrared radiation. In a first step of the process, at least one hole (5) is made in a sheet-, film-, web- or plate-shaped base body material intended to form the base body (3). In a second step of the process, the perforated base body material (6) is covered at least in the area of the hole (5) with a window film (7) fabricated from a plastic material transparent to infrared radiation and joined to the base body material (6) in an immovable manner. Subsequently, this composite body is shaped into a protective cap (1) in a third step and, where applicable, said cap is processed to the final dimensions of the protective cap (1) in a fourth step. This enables a plurality of protective caps (1) to be manufactured as on a conveyor line very simply and in no time.