1. A detection device comprising: a processor that is configured to: 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, generate a noise removal image obtained by removing a noise component from the input image; by using brightness information included in the noise removal image generated, calculate an attenuation factor of reflected light from an object captured in the input image for each pixel of the input image; generate an attenuation factor image in which the attenuation factor in each pixel is represented as brightness of the pixel; and determine whether smoke is generated in the scene to be monitored based on a brightness difference between a reference attenuation factor image and the attenuation factor image, the reference attenuation factor image being an image in which the attenuation factor, which is obtained from an image captured under a situation where smoke is not generated, is represented as brightness of the pixel in each pixel.