abstract: 'Wandering null geodesics' and 'wandering set' are defined as a generalization of photon sphere.Several characteristics of the wandering set would be revealed.This talk will shows following results about the wandering set. 1, the exisitence theorem of the wandering set 2. conjugate point of wandering null geodesics 3. formation of the (truncated) wandering set 4. global structure of the congruence of wandering null geodesics 1-3 might be simple applications of a textbook, 'general relativity by Wald'.