PURPOSE: To obtain a congestion degree detecting device of a simple consitution by providing a detecting element responding to a variation of infrared rays generated from the human body, making a field of view of the element synchronize with a scanning mirror rotating at a constant period, converting the output of the element to a signal corresponding to number of a person, to count.

CONSTITUTION: Infrared rays 14 radiated from a waiting guest 13 are made incident to a rotary type plane mirror 15 through an infrared ray lens of an optical system containing case 11. The mirror 15 is rotated by a small-sized motor 16, and a synchronizing pulse detecting part 17 sends a signal synchronizing with the rotation to a synchronizing circuit 26. The infrared rays reflected by the mirror 15 are detected by the infrared ray detecting elements 18aW18e, and are inputted to a main amplifier circuits group 22 through a pre-amplifier 20 and a band pass filter 21. A signal fetched from the circuit group 22 is inputted to a counting circuit 24 through a digital multiplexing circuit 23. The circuit 24 executes counting at every rotation of the mirror and outputs a signal showing at every rotation of the mirror and outputs a signal showing the number of the persons in a field of view. An output device 25 executes display of the number of the persons and the decision of a congestion degree, and outputs them. In this way, the congestion degree detecting device of the simple constitution is obtained.