A device, usable in proximity exposure wherein a first object having a pattern and a second object to which the pattern is to be transferred are disposed close to each other, for detecting the relative position of the first and second objects, is disclosed. The device includes a light source for projecting light to the first and second objects; a light detecting system for detecting light from one of the first and second objects illuminated by the light source; and a position detecting system for detecting the relative position of the first and second objects, on the basis of the detection by the light detecting system; wherein the light from the light source has a spectrum whose full width of half maximum is between .lambda..sup.2 /200000 nm and 60 nm, where .lambda. is the wavelength of the light.