PURPOSE: To obtain a pyroelectric type IR sensor of simple constitution having high reliability by constituting the same in such a way that a reflection plate is formed into a tray shape projecting downward in the central part, a pyroelectric film is sandwiched by the circumferential edge of the reflection plate and an earth ring, and the film is connected to a metallic case by the earth ring and to a field effect type transistor by the reflection plate.

CONSTITUTION: When IR ray is made incident to the sensor from the outside of a metallic case 27 through a window hole 26 and a window member 25, voltage is generated in a pyroelectric film 22. The IR ray transmitted through the film 22 is reflected by a reflection plate 21 and is made incident again to the film 22. The incident IR B1, C2 produce a ratio. The output of the film 22 is applied through the plate 21 to the gate of a field effect type transistor 17, and an earth circuit is grounded via an earth ring 23 to the case 27.