A technique for an infrared radiation thermometer used for thermography detects measurement abnormality of the infrared radiation thermometer and estimates the causes of the measurement abnormality such as contamination of an objective lens and a malfunction in a mechanism section of the infrared radiation thermometer. The measurement abnormality detector has a dummy lens 21 placed in the periphery of the objective lens 11 of the 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 of the received light, and determination means 50 for calculating the attenuation rate of the projected light on the basis of the quantity of the received light measured by the laser displacement meter 22, estimating the degree of contamination of the dummy lens 21 on the basis of the calculated attenuation rate, and judging, on the basis of the degree of contamination of the dummy lens 21, the necessity of warning for contamination of the objective lens 11 and measurement abnormality of the thermometer 10.