To provide a crime prevention sensor with an obstruction detecting function of high reliability capable of surely detecting the obstructing conduct in the presence of the obstructing conduct that an obstruction is attached to an outer face of an envelope member on the light entering side of a crime prevention sensor.

A light entering side envelope member displaced or deformed in receiving the pressure, is mounted on a body A having an infrared ray detecting element 4. The light entering side envelope member is composed of a lens 5 for determining a detection area B of the infrared ray detecting element 4, or a cover 28 for covering a plane of the incidence side of the infrared ray detecting element 4. Further light projecting and receiving units 11 and 12 are mounted for detecting the displacement or deformation of the light entering side envelope 5 and 28, and a detecting circuit 15 for detecting the obstructing conduct on the basis of the detection.