Noun which may take the genitive case particle 'no', Na-adjective (keiyodoshi)
natural; right; proper; just; reasonable; appropriate; deserved
彼が息子の自慢をするのは当然だ。It is only natural that he be proud of his son. Adverb (fukushi)
naturally; as a matter of course; rightly; deservedly; justly; of course
Also written 当前【とうぜん】