Preform for forming double container

A preform for forming a double container, the preform configured to form a double container including:

• an inner container configured to be deformed to reduce a capacity according to reduction in contents accommodated;
• and
• an outer container in which the inner container is installed, and which is provided with an outside air introduction hole into which an outside air is introduced between the inner container and the outer container according to reduction in the contents,
• the preform comprising:
• a bottomed cylindrical inner preform configured to form the inner container;
• and
• a bottomed cylindrical outer preform configured to form the outer container, wherein
• the inner preform is inserted into the outer preform in a state in which a mouth portion of the inner preform is fitted into a mouth portion of the outer preform,
• a longitudinal groove, extending downward from an upper end opening edge of the outer preform, is formed in an inner peripheral surface of the mouth portion of the outer preform,
• the outside air introduction hole opens in the longitudinal groove,
• a flange portion continuously extending over an entire length in a circumferential direction, abutting the upper end opening edge of the outer preform over the entire length in a circumferential direction, and closing an upper end opening of the longitudinal groove, is provided on the mouth portion of the inner preform, and
• the outside air introduction hole is disposed below and apart from the upper end opening edge of the outer preform.