The article is devoted to the theoretical problems of phenomenological sociology. The author analyses category-conceptual apparatus used by P.L.Berger and Th.Luckmann, as followers of A.Schutz. Their works are treated here as a 'post-classical' period of phenomenological sociology, although both scientists don't accept a possibility of 'phenomenological sociology' and speak about phenomenology as a proto-sociology. The analysis is focused on the concept 'legitimation', as well as and other concepts which help to understand it, such as: 'symbolic universe', 'externalization', 'objectivation', 'internalization', 'habitualization', and 'religious representations'. The author also compares Schutz's context of the term 'life-world' with that of Berger and Luckmann's concept 'world of everyday life', that has narrower definition. An integral characteristic of their concept is 'institutionalization'; they try to find mechanisms which make a life-world as taken-for-granted. In order to understand deeper this fundamental the author refers to legitimation conception and conception of symbolic universes. As a conclusion, the author makes an emphasis on the necessity of further development of theoretical level of phenomenological sociology. Regarding this task, some concepts of the theoretical systems of P.L.Berger and Th.Luckmann have an extremely high heuristic and gnoseological potential.