An attached matter detector includes: a light source that emits light to a transparent member; imaging device that receives light emitted from the light source and reflected by an attached matter on the transparent member, and consecutively images an image of the attached matter at a predetermined imaging frequency; and an attached matter detection processor that detects the attached matter based on an image imaged by the imaging device, wherein the light source emits light that flickers at a drive frequency different from the imaging frequency, and the imaging device receives the reflected light via an optical filter that selects and transmits the reflected light, and the attached matter detection processor detects a beat on an image generated by a difference between the imaging frequency and the drive frequency, and identifies an image region where the beat is detected as an attached matter image region.