A mantle tube including a bendable member, which is formed in such a way as to be received in an elongate, annular, cylindrical member, includes two fixed sites that are fixed to the cylindrical member and a bendable site that is interposed between these fixed sites and bendable in a direction intersecting a longitudinal direction thereof, an angle adjustment member that is capable of adjusting a bending angle of the bendable site, and an angle holding mechanism that is capable of holding, at any desired bending angle, the bendable site bent by the angle adjustment member.