Ikkunbanmin-ron is a theory in which constitutional authority is given to only one ruler, and under this sovereignty any discrimination and distinction from social status shold not be allowed among all other people.
At first the Meiji Government kept the class system, introducing the peerage system by transferring the samurai class, the previous ruling class, into one of the classes of the new system as Shizoku (literally, the warrior class), but the major political changes the government took, including Haihan-chiken (abolition of feudal domain and establishment of prefectures), Conscription Ordinance and Chitsuroku-shobun (measures to abolish hereditary stipends) were significant blows to the privileged class.
Meantime, as the Ikkunbanmin-ron theory was still robust as an undercurrent ideology at the court and among ordinary people, the authority of the emperor was consequently reinforced, and the privileges to the nobles other than the Imperial Family became gradually reduced. The warrior class turned out to be a mere shell in the early stage, and the peerage system, in which a good deal of privilege relating to the House of Peers was given, stirred up criticism. Some people from the peerage wondered about the privileged class system and gave up their peerage.
The Ikkunbanmin-ron theory was used as a great cause of toppling the domain clique system and introducing democracy, and it functioned as an ideology to oppose not only to the inherited privileges but also to the powerful wealthy class, most notably Zaibatsu (industrial and financial conglomerates). In the early Showa period, the theory once again got supporters, including out-of-power intellectuals, young officers and reformist bureaucrats. The theory has no longer been advocated openly since Japan lost the Second World War and was occupied by the Allied Forces accordingly.