In a signal processing apparatus including a mixing region detecting unit, and an image capturing condition updating unit, the mixing region detecting unit detects a region where a non-visible light image signal is to be mixed with a visible light image signal, the non-visible light image signal and the visible light image signal being generated by repeatedly performing imaging on a basis of a predetermined image capturing condition. The image capturing condition updating unit newly generates an image capturing condition in the non-visible light image signal on a basis of the non-visible light image signal in the detected region to update the predetermined image capturing condition. A region including fog, or the like is detected, a visible light image signal is mixed with a non-visible light image signal in the region, and image capturing conditions of the non-visible light image signal in the region are updated.