A seal structure is used in a mold constituted by a plurality of segments to manufacture a hollow composite material structure. In the seal structure, a seal member formed in a line shape having ends is inserted in a seal groove provided on at least one of adjacent side surfaces of the segments. A lip portion is provided at a seal upper portion of the seal member such that at least a tip end of the lip portion projects to an outside of the seal groove. When the adjacent side surfaces of the segments are coupled to each other, the seal member is crushed in a cross sectional direction by the other of the adjacent side surfaces of the segments. With this, complication of the manufacture of the composite material structure can be suppressed or avoided while realizing a satisfactory sealed state between the adjacent segments.