A method of making a gap between first and second objects have a predetermined value includes steps of introducing light to an entry window on the first object, detecting an intensity of light from an exit window on the first object, and positioning the second object, with respect to a direction concerning the gap, based on an intensity of light detected in the detecting step. The exit window is positioned so that light enters through the entry window of the first object, reflects off the second object, and then enters the exit window of the first object if the gap has the predetermined value.