A detection device that detects a mark provided on the back side of an object, the detection device includes a first detection unit configured to detect the mark from a surface side of the object; a second detection unit configured to detect a surface position of the object; and a processing unit. The processing unit determines a thickness of the object based on a difference between a first focus position acquired with reference to the position of the mark detected by the first detection unit and a second focus position acquired with reference to the surface position detected by the second detection unit.