A technique for an infrared radiation thermometer used for thermography detects abnormal measurement of the infrared radiation thermometer and estimates the causes of the abnormal measurement such as contamination of an objective lens and a malfunction in a mechanism section of the infrared radiation thermometer. The abnormal measurement detector has a dummy lens (21) placed in the periphery of the objective lens (11) of the infrared radiation thermometer (10) so as to be at a position and in an attitude that are more susceptible to contamination than the objective lens (11), a laser displacement meter (22) for projecting light to the dummy lens (21) at each predetermined time or at a predetermined timing, receiving the light reflected by the dummy lens (21), and measuring the quantity ofthe received light, and determination means (50) for calculating the attenuation rate of the projected light based on the quantity of the received light measured by the laser displacement meter (22),estimating the degree of contamination of the dummy lens (21) based on the calculated attenuation rate, and judging, based on the degree of contamination of the dummy lens (21), the necessity of warning for contamination of the objective lens (11) and abnormal measurement of the infrared radiation thermometer (10).