1. A detection apparatus which detects an original mark on an original or an original reference member and a substrate mark on a substrate or a substrate reference member, the original mark and the substrate mark being arranged via a projection optical system, the apparatus comprising: an optical system including an imaging device and configured to form an image of the original mark and an image of the substrate mark onto the imaging device, wherein the optical system includes a detection reference member having a first mark and a second mark, and is configured to: form an image of the first mark onto the original or the original reference member, and form an image of the second mark onto the substrate or the substrate reference member via the projection optical system and the original or the original reference member, and form an image of the original mark, an image of the substrate mark, an image of the first mark, and an image of the second mark onto the imaging device.