Object: Gas turbine blade cooling is generally made by use of air, which has a limitation in enhancement of a thermal efficiency, so steam as a cooling medium is being studied. In a steam cooling method, however, a secure recovery of cooling steam and a secure prevention of leakage is required. The present invention provides a new unit which can satisfy such requirement. Solving means: There are provided a column extending in a radial direction on an outer periphery of a shroud, a supply passage and a recovery passage of a cooling medium within said column and a means for joining a supply tube and a recovery tube of the cooling medium connected to a starting end of said supply passage and a finishing end of said recovery passage, respectively, at a terminal end portion of said column, thereby the cooling medium supplied via the supply tube and the supply passage is recovered securely via the recovery passage and the recovery tube so that no leakage occurs outside, thus an employment of steam etc. as a cooling medium can be realized.