A theoretical analysis is presented of spontaneous pattern formation in a system consisting of a photorefractive medium and a feedback mirror. A threshold condition for the pattern formation is derived from a new set of coupling equations with the assumption that reflection gratings are dominant. It is shown that the pattern formation can occur with only energy coupling. The direction of the sideband beams is calculated as a function of the mirror position, and the result is compared with that of an experiment with a KNbO3 crystal.