A preform for molding a dual container, in which an inner preform is inserted into an outer preform in a state in which a mouth portion of the inner preform is fitted into a mouth portion of an outer preform, the inner preform is formed of a crystalline resin, a crystallized region is provided in at least a portion of the inner preform, the crystallized region having a degree of crystallization greater than that of the other portion, the portion of the inner preform adjacent to the mouth portion from below the mouth portion and located below the outside air introduction hole, a step portion facing upward and a rib extending upward from the step portion are formed on an inner circumferential surface of the inner preform, and at least a part of the rib is adjacent to the crystallized region from above the crystallized region.