A seal member formed in a line shape having ends is inserted in a seal groove formed on at least one of adjacent side surfaces of segments. A lip portion is provided on a seal upper portion of the member so at least a tip end thereof projects to an outside of the seal groove when the member is inserted in the groove. The seal member is hollow or includes a concave portion on at least a seal bottom portion thereof. When assembling the segments as a single structure, the adjacent side surfaces of the segments are coupled to each other, and the seal member is crushed in a cross sectional direction by the side surface of the other segment to seal between the segments. With this, complication of manufacturing steps can be suppressed or avoided while realizing a satisfactory sealed state between the adjacent segments of a mold.