To perform the fixing of an uppermost-part supporting grating simply without performing a swaging process.

A sleeve 8, wherein a thimble tube 2 is inserted. is made to extrude from a thimble cell 6 of an uppermost-part supporting grating 3a to the vicinity of an upper nozzle plate 7. A protruding part 9, which is protruding to the outer direction or a radius is formed, at the outer surface part of the tip of the sleeve 8. A recessed part 10, which is externally coupled with the protruding part 9, is formed at the inner surface part of the lower part. Furthermore, a tubular collar 12, where the lower part is formed so that the part can be expanded, is fixed and disposed at the upper part of the thimble tube 2. The recessed part 10 of the collar 12 and the protruding part 9 of the sleeve 8 are coupled. The collar 12 is inserted to a thimble-tube inserting hole 13 of the upper nozzle plate 7. The upper nozzle plate 7 is tightened by a flange part 14 formed at the lower part of the collar and a nut 16, which is screwed into the upper part of the collar.