A shaft seal mechanism (leaf seal) by which a desired seal performance can be stably obtained is provided. The shaft seal mechanism comprises a concave groove 71 formed larger than a minimum size of a plate width of thin plates 29 and a fitting piece 61 fitted in a gap between the concave groove 71 and a leaf seal 25. The leaf seal 25 comprises an annular thin plate assembly 29A constructed by the thin plates 29 and a plate spring 56 integrally formed therewith supporting the annular thin plate assembly 29A to be levitated coaxially with a rotating shaft 23. The thin plates 29 have their outer circumferential proximal end side made movable relative to the concave groove 71. A shaft seal mechanism assembling structure is also provided comprising a pressure receiving surface 61b and a lower pressure side plate 54 forming a predetermined gap size between the pressure receiving surface 61b and one side edges of the thin plates 29.
