An assembled mandrel (11;21;31;41) for forming a hollow structure having a shape similar to that of the assembled mandrel (11;21;31;41), the assembled mandrel (11;21;31;41) comprising: a first mandrel member that extends in a longitudinal direction; a second mandrel member that extends in the longitudinal direction, is adjacent to the first mandrel member, and is adapted to be separated from the first mandrel member and to be withdrawn in the longitudinal direction after the forming of the hollow structure; and either a) a planar detachable member (5;7) that is disposed to cover at least one surface of facing contact surfaces between the first mandrel member and the second mandrel member, wherein a plurality of the detachable members that are formed in the shape of a strip and are arranged so as to perform covering in parallel and so as to extend in the longitudinal direction, and empty spaces that extend in the longitudinal direction are provided between the respective detachable members, or b) a resin strip that is sandwiched between the first mandrel member and the second mandrel member via a mandrel-releasing agent, extends in the longitudinal direction, and is made of the same kind of resin material as a prepreg that is intended to be layered on the assembled mandrel to form the hollow structure. a first mandrel member that extends in a longitudinal direction; a second mandrel member that extends in the longitudinal direction, is adjacent to the first mandrel member, and is adapted to be separated from the first mandrel member and to be withdrawn in the longitudinal direction after the forming of the hollow structure; and either a) a planar detachable member (5;7) that is disposed to cover at least one surface of facing contact surfaces between the first mandrel member and the second mandrel member, wherein a plurality of the detachable members that are formed in the shape of a strip and are arranged so as to perform covering in parallel and so as to extend in the longitudinal direction, and empty spaces that extend in the longitudinal direction are provided between the respective detachable members, or b) a resin strip that is sandwiched between the first mandrel member and the second mandrel member via a mandrel-releasing agent, extends in the longitudinal direction, and is made of the same kind of resin material as a prepreg that is intended to be layered on the assembled mandrel to form the hollow structure. a) a planar detachable member (5;7) that is disposed to cover at least one surface of facing contact surfaces between the first mandrel member and the second mandrel member, wherein a plurality of the detachable members that are formed in the shape of a strip and are arranged so as to perform covering in parallel and so as to extend in the longitudinal direction, and empty spaces that extend in the longitudinal direction are provided between the respective detachable members, or b) a resin strip that is sandwiched between the first mandrel member and the second mandrel member via a mandrel-releasing agent, extends in the longitudinal direction, and is made of the same kind of resin material as a prepreg that is intended to be layered on the assembled mandrel to form the hollow structure.