PURPOSE: To provide an infrared ray chopper for performing signal processing of the output signal of a two-dimensional infrared ray array sensor with a simple circuit configuration and an inexpensive signal processing means.

CONSTITUTION: An infrared ray chopper 1 is provided with a shutter peripheral wall 7 for cutting infrared rays formed on a cylindrical surface and a rotary means 6 for turning the shutter peripheral wall 7 around a rotary axis, and an infrared ray array sensor 4 where picture elements 3 are arranged two- dimensionally is provided opposite to the inner wall surface of the shutter peripheral wall 7. Four opening windows 8A-8D for transmitting infrared rays for one picture element are formed on the shutter peripheral wall 7. Whole picture elements 3 of each row are formed while the position is shifted at a section pitch where the scanning is completed so that, for example, the opening windows 8A-8D are matched to each row array position of the picture element 3 of each corresponding sensor 4, and the opening window 8B starts scanning a second row when the opening window 8A has compled scanning of the first row when the shutter peripheral wall 7 is turning.