1. An attached matter detector comprising: a light source that emits light toward a transparent member; an imager that receives transmitted light transmitted through the transparent member from a predetermined imaging region on a light-receiving region for a predetermined imaging region and images an image of the imaging region, and receives reflected light on a predetermined light-receiving region for attached matter detection when light emitted from the light source is reflected by attached matter attached on the transparent member and images an image of the attached matter; a parameter calculator of an image for an attached matter image region that calculates a parameter of an image for an attached matter image region as image information of an image region of the attached matter imaged by the imager; a parameter calculator of an image for an imaging region that calculates a parameter of an image for an imaging region as image information of the imaging region; and an output device of an attached matter state signal that outputs a signal that shows a state of the attached matter based on the calculated parameter of the image for the attached matter image region and the calculated parameter of the image for the imaging region.