A reinforcing member made from a fibrous material and disposed at least on one face of a strip-like expanded graphite to form a strip-like base member. The base member is stranded to be formed as a cord-like member. At this time, one side end edge of the base member is placed on the outer peripheral surface of the cord-like member, and, in the side end edge, the strip-like expanded graphite is more elongated in the width direction than the reinforcing member. The stranding of the base member is applied while the strip-like expanded graphite which is elongated in the width direction is placed on the inner side, and the reinforcing member which is short in the width direction is placed on the outer side. According to the configuration, both the reinforcing member and the strip-like expanded graphite are placed in a spiral manner to be alternately arranged in the axial direction on the outer peripheral surface of the cord-like member.