A composition capable of radical polymerization and air are accommodated in a packaging bag prepared using a heat- sealable synthetic resin sheet. In a preferred form, the packaging bag has at least one layer that bars the passage of at least one of a solvent, a gas, visible light and ultraviolet light, such as a metal layer. For example, it is prepared from a sheet of 5-layer laminate structure consisting of, arranged from inside toward outside in sequence, synthetic resin sheet (1) / adhesive layer (2) / synthetic resin sheet (3) / metal layer (4) / synthetic resin sheet (5). The volume of air in the packaging bag is preferably in the range of 1 to 50% based on the volume of composition capable of radical polymerization, and the total thickness of sheets used in the packaging bag is preferably 50 to 300 μm. The thickness of the metal layer is preferably 5 μm or more. This packaging form is most suitable for a composition capable of radical polymerization, such as a resist ink, because not only can the aging stability of composition capable of radical polymerization during storage be ensured but also the safety during transportation can be ensured.