This invention relates to a technique for hermetically sealing an artificial leg including a cylinder device. A shell structure surrounds the outside of the cylinder device. The cylinder device is not entirely received within the shell structure, but a rod member extends to the outside of the shell structure from the cylinder body within the shell structure and a part of the rod member is located outside the shell structure. In this invention, the rod member, which extends from the inside to the outside of the shell structure, is hermetically sealed at the periphery thereof by a bellows packing member. The bellows packing member comprises an inner bead, an outer bead which concentrically surrounds the outside of the inner bead, and a bellows packing body part which connects the inner and outer beads. The bellows packing body part includes corrugations, and each corrugation is less curved than an imaginary circle formed between the outer bead and the inner bead and is configured in such a manner that both ends of the corrugation reach the outer bead or are located at positions closer to the outer bead than the inner bead.
