1. A detection device comprising: an extraction unit that, by using a result of an image analysis of an input image that is a captured image obtained by capturing a scene to be monitored, generates an image obtained by removing a noise component from the input image; a calculation unit that, by using brightness information included in the image generated by the extraction unit calculates an attenuation factor of reflected light from an object captured in the input image; and a detection unit that determines whether smoke is generated in the scene to be monitored based on the attenuation factor.