PURPOSE: To provide an inexpensive and compact infrared ray sensor with a simple manufacturing process.

CONSTITUTION: An infrared ray detector 15 is provided on a silicon substrate 13 and a chopper 40 is provided at the infrared ray incident side of the upper side of the infrared ray detection part 15. An infrared ray absorption region 11 and an infrared ray reflection region 1 are alternately provided on the light- reception surface of the infrared ray detection part 15, and a transmission part 25 and a non-transmission part 24 are provided in the same direction as the installation direction on that light-reception surface at the chopper 40. The chopper 40 is moved relatively in the direction A for the infrared ray detection part 15 to alternately cross the infrared ray absorption region 11 and the infrared ray reflection region 1, and it absorbs infrared rays through the transmission part 25 for detecting by an infrared ray detection film 16 only when the infrared ray absorption region 11 opposes the transmission part 25 of the chopper 40, thus infrared rays are intermittently detected by the infrared ray detection part 15.