The invention is directed to a protective cap (2) for a temperature measurement probe (30) of an infrared radiation thermometer (1) that can be introduced into a body cavity (31). The protective cap (2) is comprised of a base body (12) shaped to fit the body cavity (31) and having a window (15) transparent to infrared radiation. The base body (12) is provided with additional structures (13; 18, 20) at least in parts to improve heat insulation between the temperature measurement probe (30) and the body cavity (31). This heat insulation of the temperature measurement probe (30) prevents measurement errors by the infrared radiation thermometer (1).