An infrared intrusion detector uses infrared-sensitive sensors with pyroelectric sensor elements for detecting infrared radiation from a spatial region to be monitored. Infrared radiation passes through an entrance window and reaches the sensor elements via focusing mirrors. Extraneous radiation, outside the useful radiation band, is eliminated by filtering at the entrance window and by an optical transmission filter, and by scattering at suitable rough surfaces of the focusing mirrors. As a result, the infrared intrusion detector is less sensitive to extraneous radiation and less likely to produce false alarms.