Flexible package

A flexible package, comprising:

• a first side-surface film, a second side-surface film, and a bottom film which are joined together, the bottom film being inserted between the first side-surface film and the second side-surface film so as to be folded in half and inserted from a fold line side, with a predetermined insertion length;
• and
• a storage part formed by sealing peripheral portions of the films, the storage part having a lower bottom surface which is the bottom film,
• wherein
• on one or both of two side edge portions which are sealed regions of the peripheral portions at both side ends of the first side-surface film and the second side-surface film, a non-sealed region is provided which is surrounded by the sealed regions of the first side-surface film and the second side-surface film and is not sealed over a predetermined length,
• in the non-sealed region, a gas injection portion is formed so as to contain a gas whose specific heat at constant volume is 0.67 kJ/kg·deg or higher at 0° C. and 1 atm,
• the gas injection portion has a diameter not smaller than 3 mm but not larger than 50 mm,
• a repelling force, measured when the entirety of the gas injection portion is nipped from a first side-surface film side and a second side-surface film side and squeezed until the width of the nipped gas injection portion becomes half the diameter of the gas injection portion, is not smaller than 7 N but not larger than 30 N at 23° C. and 1 atm,
• the non-sealed region has an oxygen permeability equal to or lower than 30 cc/(m2·day·atm) at 20° C. and 60% RH,
• the side edge portions each have a seal strength equal to or larger than 30 N/15 mm,
• a loop stiffness of the first side-surface film and the second side-surface film in a direction corresponding to a vertical direction when the flexible package is made to stand by itself is not smaller than 30 mN/25 mm (width) but not larger than 1300 mN/25 mm (width), and
• a loop stiffness of the first side-surface film and the second side-surface film in a direction corresponding to a horizontal direction when the flexible package is made to stand by itself, is not smaller than 20 mN/25 mm (width) but not larger than 1200 mN/25 mm (width).