A detection system on detecting a light source or an object irradiated by the light source serving as an object to be photographed with high accuracy and capturing a vivid image of the object to be photographed and a signal processing method and a smoke sensor. A detection system includes an image pickup unit, a light source, a first computing unit, a second computing unit, a third computing unit, a detecting unit, and a correction instructing unit. The correction instructing unit outputs a correction instructing signal when the absolute value of a computation result A or that of a computation result B exceeds a permissible decrement. The third computing unit corrects the computation result of which absolute value is reduced between the computation results such that the decrement is less than or equal to the permissible decrement when receiving the correction instructing signal, and performs computation.