A detection device includes: an imaging unit that forms an image of an incident beam on an image detection unit; an emission unit that includes a guided portion that is guided to a guiding portion provided at the imaging unit and guiding an attachment-detachment operation of the guided portion, a positioned portion that is positioned in a positioning portion provided at the imaging unit, and an emission member that emits a beam toward a direction of a transportation path such that a beam reflected from the medium is incident to the imaging unit; and a setting unit that includes a positioned portion that is positioned in a positioning portion provided at the emission unit when a drawing unit is pressure-inserted into the image forming apparatus and a setting surface that sets a position on the medium at which the beam is reflected.