Double container, inner container, and outer container

An inner container installed inside an outer container, the inner container comprising:

• a jointed portion configured to join to a jointing portion faulted in the outer container to prevent the inner container from being separated from the outer container, the jointed portion including a flange formed on a neck portion of the inner container and configured to be joined to a hook portion of the jointing portion of the outer container;
• and
• a second engaging portion of the inner container engaged with a first engaging portion formed in the outer container so as to prevent the inner container from rotating relative to the outer container, the second engaging portion including a rib and configured to be engaged with one of a plurality of recesses of the outer container,
• wherein a wall thickness of the neck portion of the inner container is 0.5 mm to 4.0 mm in a direction perpendicular to a longitudinal direction along a longest side of the inner container, and a wall thickness of a portion other than the neck portion of the inner container is 0.05 mm to 0.3 mm in the direction perpendicular to the longitudinal direction along the longest side of the inner container.