1. In an apparatus for detecting a position of a first mark formed on a substrate in accordance with an image signal resulting from the detection of said first mark and a second mark formed on an index plate arranged at a position which is substantially conjugate to said substrate with respect to an objective optical system including an image pickup device, the improvement comprising: (a) an image optical system whereby said index plate and said image pickup device are arranged in a substantially optical conjugate relation to each other with respect to said imaging optical system; (b) first illumination system, that produces an illuminating light not including reflected light from said substrate for illuminating a first area containing said second mark on said index plate; (c) second illumination system optically connected to said objective optical system for substantially vertically illuminating through said objective optical system a localized area of said substrate corresponding to an area of said index plate other than said first area, said second illumination system illuminating at least said localized area on said substrate by a light of a wide band of wavelengths; and (d) said image pickup device for detecting an image of said first mark formed on said substrate and an image of said second mark formed on said index plate and generating a corresponding image signal when said first mark on said substrate is positioned within said localized area.