Double container and manufacturing method therefor, preform, and container manufacturing method

A method of manufacturing a double container, wherein:

• the double container comprises a container body;
• the container body has an outer shell and an inner bag;
• the inner bag is contractable;
• the outer shell comprises an outside air introduction hole for introducing outside air between the outer shell and the inner bag;
• the method comprises a heating step and a molding step;
• in the heating step, a preform configured by covering an inner preform with an outer preform is heated for softening and is put into a softened state;
• the molding step comprises a blow molding step of blowing air into the inner preform in the softened state;
• in the heating step, the preform is heated by a plurality of heaters with the preform being rotated;
• the plurality of heaters are arranged at a position adjacent to a side surface of the preform to align along a longitudinal direction of the preform;
• a part of the preform at which the outside air introduction hole is formed is defined as an outside air introduction hole formation part;
• among the plurality of heaters, a heater closest to the outside air introduction hole formation part is defined as a closest heater;
• among the plurality of heaters, heaters other than the closest heater are defined as other heaters;
• V is defined as a value of an output of the closest heater divided by a square of a distance from the closest heater to the outside air introduction hole formation part;
• Wa is defined as a maximum value among values obtained by dividing each output of the other heaters by a square of a distance from each of the other heaters to the preform;
• and
• V/Wa is 0.60 or less.