Abstract: To make a ghost image that is caused by a step place between a lens and a back plate inconspicuous in a configuration in which an optical axis of an infrared camera is disposed tilted relative to a normal direction of a plate face of a mirror element. A flat surface portion and a recessed surface portion are formed on the back plate disposed behind the mirror element. At least a part of a lens exposure opening is formed in an inclined surface of the recessed surface portion, the inclined surface being inclined relative to the flat surface portion. The lens of the infrared camera is disposed on the inclined surface with the optical axis titled relative to the normal direction of the flat surface portion, following the lens exposure opening.