PURPOSE: To provide a temperature detecting device which can prevent the influence of heat rays from the outside of its monitoring area and, at the same time, the position of which can be easily adjusted.

CONSTITUTION: A first aperture 4 is provided at a comparatively long distance from a pyroelectric sensor 11 having a light receiving section 2 which receives heat rays from the outside at its central part on one surface of a pyroelectric substrate 1 and, at the same time, a second aperture 5 is provided at a comparatively short distance from the sensor 11. By means of the apertures 4 and 5, the light receiving area α with which heat rays from a monitoring area A collide is made smaller than the area of the section 2 and, at the same time, the collision of heat rays from the outside of the area A with the area other than the area α is prevented. Therefore, not only an excellent visual field output characteristic can be obtained, but also the yield and working efficiency of this temperature detecting device can be improved.