The invention relates to an infrared intrusion detection device. It comprises a housing 1, a window 2, which is arranged at a front side of the housing and which is transparent for at least infrared radiation emitted by an intruder, an infrared sensor arranged inside the housing for detecting the infrared radiation emitted by the intruder. Further it comprises a light guide 7, which is arranged inside the housing, which has at least one light entrance facet and which has a plurality of light exit facets 3 being arranged at the front side of the housing. A light emitter 6 is arranged inside the housing for injecting light into the at least one light entrance facet of the light guide. A light detector is provided for detecting light reflected back into the housing by an obstacle in the vicinity of the window. A signal processing unit is arranged for triggering an alarm, when an absolute difference of the intensity of back-reflected light to a predetermined value exceeds a threshold value.