PURPOSE: To obtain the detection apparatus which is hardly affected by an irregularity in the output signal of an infrared sensor an which avoids a malfunction when a plurality of small animals creep into a monitoring region.

CONSTITUTION: A CPU 5 acquires a thermal image at (m)×(n) pixels via a pyroelectric infrared sensor 2, an impedance converter 3 and an A/D converter 4. Then, a window is set in an initial position on the thermal image, and the occupation ratio of a heat-radiating object inside the window is investigated. When the occupation ratio is, e.g. 90 % or higher, the heatradiating object is judged to be not a small animal but a human being, this judgment is left in an alarm 8, and an extrance is opened only for a definite time by an entrance opening and shutting device 10.