The present invention provides a positioning mechanism and an exposure apparatus that can be used in a vacuum ambience and has a damping function with small degassing and small dust creation. A positioning mechanism according to the present invention, when used with first and second members, serve to position the second member with respect to the first member, wherein the positioning mechanism include a positioning portion capable of relatively positioning the second member relative to the first member, and an attenuating material provided at least at a portion of a periphery of the positioning portion.