A scan type exposure apparatus for transferring an image of a pattern formed on a mask onto a substrate by synchronously scanning the mask and the substrate in a scanning direction. The apparatus includes a movable stage for holding a substrate thereon, an optical system through which an exposure light path extends, a cover member for encircling the exposure light path, from an end of the optical system toward the stage, and a gas supplying port for discharging an inactive gas into the cover member, wherein the gas is supplied at a predetermined angle with respect to the scan direction.