Abstract: The piston boot is first set at the middle portion of a cylinder jig having a small diameter upper step portion and a large diameter lower step portion with the middle portion position therebetween and having radial air blow off vents. The upper step portion is then inserted into an inner circumferential face of a piston while simultaneously advancing the lower step portion toward an end face of the piston from which the inner circumferential face extends and blowing air from the blow off vents, as a result of which the piston boot is radially expanded and fitted on an outer circumference of the piston adjacent the end face. A cup form jig slidably fitted around the lower step portion is then advanced toward the piston boot and further advances the piston boot until the piston boot is fitted into a piston boot fitting portion of the piston.