To provide an infrared radiation detector capable of simplifying the manufacturing process, reducing the cost, shortening the manufacturing time, and improving the yield.

On a substrate 1, an infrared radiation reflecting film 2 and an insulating retainer structure layer 3 having infrared radiation absorbing effect are laminated, and a heat-sensitive resistor film 5 and an electrode 4 are formed. An insulating retainer layer 9 is formed on the whole surface of the substrate 1. An aperture part 6 is formed in the insulating retainer layers 3, 9. A cavity part 8 is formed by anisotropic etching from the aperture part 6. The retainer layers 3, 9 are amorphous silicon nitride films formed by holding a substrate temperature during film formation at a temperature higher than or equal to a room temperature and at most 400°C, and reactive-sputtering a silicon target at a pressure of at least 0.7 Pa and at most 1 Pa in a mixed gas of argon and nitrogen. The total thickness of the layers 3, 9 is at least 1 μm and at most 4 μm.