Double container

1. A double container that has an outer layer body and an inner layer body located on an inner side of the outer layer body and mainly made of a polyester resin and is formed by biaxial stretch blow molding, the double container comprising:

a tubular mouth portion;

a shoulder portion connected to a lower end of the mouth portion, gradually increasing in diameter in a downward direction, and having an outward protruding curved shape over a length in a vertical direction in a longitudinal section;

a barrel portion connected to a lower end of the shoulder portion; and

a bottom portion closing a lower end of the barrel portion,

wherein the mouth portion has an outside air introduction port for introducing outside air between the outer layer body and the inner layer body,

the mouth portion is formed by an outer mouth portion of the outer layer body and an inner mouth portion of the inner layer body,

the mouth portion comprises an upper tube in contact with an inner surface of the outer mouth portion, an inclined tube decreasing in diameter in the downward direction from the lower end of the upper tube, and a part suspended from the lower end of the inclined tube,

the inner mouth portion comprises a projection piece extending from an upper end of the inclined tube in the downward direction such that a space for air passage can be maintained between the outer layer body and the inner layer body around and below the outside air introduction port, and

the double container comprises a shoulder rib that is concave or convex as seen from outside and at least extends from an upper end of the shoulder portion to the lower end of the shoulder portion.

2. The double container according to claim 1 , wherein a plurality of the shoulder ribs are radially arranged in a top view.