A detection apparatus includes a transmission member that is provided facing a conveying path on which a medium is conveyed and that transmits light from the medium being conveyed on the conveying path; a detection section that detects the medium or an image on the medium according to the light which is transmitted by the transmission member, wherein the light is received by a light-receiving member of the detection section; and an opposing member provided on an opposite side of the conveying path from the transmission member and having at least one opposing surface that faces the transmission member. In a conveying direction of the medium, a length of the opposing surface is shorter than a length of a detection surface.