The notion of universally saturated morphisms between saturated log schemes was
introduced by Kazuya Kato. In this paper, we study universally saturated morphisms
systematically by introducing the notion of saturated morphisms between integral log
schemes as a relative analogue of saturated log structures. We eventually show that a
morphism of saturated log schemes is universally saturated if and only if it is
saturated. We prove some fundamental properties and characterizations of universally
saturated morphisms via this interpretation.
