An infrared sensor which is provided with an infrared detector 43 for converting infrared light to an electric signal and an optical element 44 for guiding the infrared light to the infrared detector 43, and in which the optical element 44 is a planar optical waveguide provided with a diffraction grating and is formed on a substrate 41 side by side with the infrared detector 43. The infrared light incident on the optical element 44 from a direction vertical to the substrate surface is deflected and guided by the optical element 44 in parallel to the substrate surface for incidence on the infrared detector 43. The infrared sensor is small in size, highly sensitive, fast in responsivity, and easy to manufacture.