An exposure device that draws a pattern on a substrate by shining a beam from a light source device on substrate and scanning the beam in a main scanning direction while varying the intensity of beam according to pattern information, including: a scanning unit having a beam scanning unit that includes a polygonal mirror whereby the beam is oriented to scan the beam, and light detector for photoelectric detection of reflected light generated when beam is shined on substrate; an electro-optical element for controlling the beam's intensity modulation according to pattern information such that at least part of second pattern to be newly drawn is drawn on top of at least part of first pattern formed on substrate; and a measurement unit measuring relative positional relationship between the first and second pattern on the basis of a detection signal output by the detector while second pattern is drawn on substrate.