To provide a device warning the presence of a person or animal in an area such as a vehicle rear part a driver cannot visually confirm to a driver's seat.

An infrared ray detector, particularly, a pyroelectric infrared ray detector transmits a noticeable sensing output when an object to be detected (person or small animal) enters and leaves a number of effective sensitivity areas set by a lens. Accordingly, a plurality of infrared ray detectors are arranged as far as possible so that the effective sensitivity areas are crossed. Namely, the infrared ray detectors are rationally arranged according to the characteristics of the infrared ray detectors.