Abstract: According to one embodiment, a passive infrared sensor 1 includes an infrared sensing element 4, a pair of lenses 5R and 5L, and main mirrors 22R and 22L. A submirror 24R is disposed on the main mirror 22R. The submirror 24R is configured so that, with respect to the main mirror 22R, the orientation of the submirror 24R is changed by a first predetermined angle in regard to the horizontal direction in a direction away from an attachment surface 30 at the installation time, and the orientation of the submirror 24R is changed by a second predetermined angle in regard to the vertical direction in a direction downward at the installation time. A submirror 23L is disposed on the main mirror 22L.