A flexible package including a package body including a first side-surface film and a second side-surface film sealed in a peripheral portion. The package body has sealed portions formed in side edge portions such that a non-sealed portion is formed in at least one of the sealed portions. The non-sealed portion has a gas injection portion containing a gas which has a specific heat at constant volume of 0.67 kJ/kg·deg or higher at 0° C. and 1 atm. The gas injection portion has a diameter of from 3 to 50 mm and is formed such that a repelling force is from 4 to 30 N at 23° C. and 1 atm when an entire gas injection portion is nipped from both sides of the first and second side-surface films and squeezed until a nipped gas injection portion has a width equal to a half of the diameter of the gas injection portion.