A package has a package body formed by mating film materials and sealing peripheral edge portions of the mated film materials so as to form a content accommodation compartment inside the sealed peripheral edge portions, the sealed peripheral edge portions being subjected to an easily peel-off treatment so that the sealed peripheral edge portions are separated and peeled off from each other when the package is opened. A seal opening starting portion is formed outside of one of the sealed peripheral edge portions so that the film materials are separated from each other at the seal opening starting portion, and at least one projection is provided on at least one of inner and outer side surfaces of the film materials forming the seal opening starting portion.