A synthetic resin preform that includes an outer body and an inner body laminated on an inner surface of the outer body and that is used for stretch blow molding, the synthetic resin preform comprising: a tubular mouth; a trunk positioned below the mouth; and a bottom that closes a lower end of the trunk, wherein in the mouth, the inner body includes a large-diameter portion provided at an upper end thereof, and a small-diameter portion continuously extending below the large-diameter portion via a connecting portion, in the mouth, the outer body is provided with an ambient air introduction hole passing through the outer body, and a neck ring positioned within a height range of the small-diameter portion, the inner body is made of a crystalline resin, and a crystallized region is formed in part of the inner body, the crystallized region having a higher degree of crystallization than other regions, the crystallized region has an upper end that is positioned within a height range extending between an upper end of the connecting portion and an upper end of the neck ring, and the crystallized region has a lower end that is positioned below the neck ring, characterized by that one of an outer surface of the large-diameter portion and the inner surface of the outer body is provided with an annular projection (14f1), another one of the outer surface of the large-diameter portion and the inner surface of the outer body is provided with an annular groove (14e1) that faces the annular projection (14f1), and the inner body is configured to be positioned relative to the outer body in a vertical direction by engagement between the annular projection (14f1) and the annular groove (14e1). a tubular mouth; a trunk positioned below the mouth; and a bottom that closes a lower end of the trunk, wherein in the mouth, the inner body includes a large-diameter portion provided at an upper end thereof, and a small-diameter portion continuously extending below the large-diameter portion via a connecting portion, in the mouth, the outer body is provided with an ambient air introduction hole passing through the outer body, and a neck ring positioned within a height range of the small-diameter portion, the inner body is made of a crystalline resin, and a crystallized region is formed in part of the inner body, the crystallized region having a higher degree of crystallization than other regions, the crystallized region has an upper end that is positioned within a height range extending between an upper end of the connecting portion and an upper end of the neck ring, and the crystallized region has a lower end that is positioned below the neck ring, characterized by that one of an outer surface of the large-diameter portion and the inner surface of the outer body is provided with an annular projection (14f1), another one of the outer surface of the large-diameter portion and the inner surface of the outer body is provided with an annular groove (14e1) that faces the annular projection (14f1), and the inner body is configured to be positioned relative to the outer body in a vertical direction by engagement between the annular projection (14f1) and the annular groove (14e1).