All injection mold for forming an automobile bumper for use on an automobile body has a first cavity region defining a thicker region having a relatively large wall thickness, a second cavity region defining a gradually varying region having a gradually varying wall thickness, a third cavity region defining a thinner region having a relatively small wall thickness and a film gate for successively introducing a molding resin into the first, second and third cavity regions. The thicker region, the gradually varying region, and the thinner region are integrally formed with each other and arranged successively downwardly when mounted on the automobile body. The gradually varying region has a wall thickness reduced at a constant rate from the wall thickness of the thicker region to the wall thickness of the thinner region in a direction from the thicker region toward the thinner region.