To reduce work processes, facilitate production and achieve weight reduction and improved shock resistance by integrally forming a bag body in a box shape using a synthetic resin material instead of cardboard material as core material.

A bag includes a box-shaped bag body 1 and a box-shaped lid member 11 which is openable/closable with hinge means 5. The bag body 1 and the lid member 11 which serve as core materials by being integrally formed in a substantially box shape using synthetic resin materials with a predetermined thickness. The outer and inner peripheral surfaces of the bag body 1 are covered with a body outer cover 2 and a body inner cover 3, respectively. The outer and inner peripheral surfaces of the lid member 11 are covered with a lid outer cover 12 and a lid inner cover 13, respectively. The body outer cover 2, the lid outer cover 12, the body inner cover 3 and the lid inner cover 13 are formed in a square bag shape so that the body outer cover 2 and the lid outer cover 12 cover the outer surfaces of the bag body 1 and the lid member 11 respectively, and the body inner cover 3 and the lid inner cover 13 cover the inner surfaces thereof respectively. The body outer cover 2 and the lid outer cover 12 are inwardly folded at opening edges of the bag body 1 and the lid member 11 respectively, and are fixed to the respective inner side faces of the opening edges. A reinforcing core material 7 is provided on a whole area of the periphery along the opening edge of the bag body 1.