To accurately detect the movement degrees in a specific moving area of an object emitting infrared such as the degree of movement of workers in a clean room with a simple constitution by using a pyroelectric sensor which senses infrared rays.

Three pyroelectric sensor 1, 1 and 1, including a part of moving area A of an object-emitting infrared rays in a sensing area 10, are attached on the circumference of a rotary plate 20 with given sensing areas 10, 10 and 10, and the rotary plate 20 is rotated with transmission from a motor 21. The sensing areas 10, 10 and 10 of each pyroelectric sensor 1, 1 and 1 are altered in three stages, and the sensed results obtained in the levels is given to a moving degree operator 3 and moving degree is obtained in the constitution.