The present invention relates to an exposure device for forming predetermined patterns onto an object. The exposure device includes an optical source for emitting a ray, an aperture member including first and second opening windows and a detection window, first and second reflective optical elements for reflecting the first and second light beams that have passed through the first and second opening windows, respectively, and an optical sensor for detecting intensity of the ray from the optical source which has passed through the detection window, the optical sensor being placed close to an area between the first and second reflective optical elements.