PURPOSE: To firmly fix an inner cylindrical body for a long period of time by a method wherein the inner cylindrical body is fixed by screwing to the inside of an outer cylindrical body so that the height thereof can be adjusted freely, and a pressing device provided to the inner cylindrical body is pressed against and fixed to a groove-like recessed part formed to the inner circumferential face of the outer cylindrical body.

CONSTITUTION: The height of an inner cylindrical body 4 is made the same as a necessary ground height by turning the inner cylindrical body 4 and by moving it up and down. Then the inner cylindrical body 4 is turned round so that a frame part 10 provided to the inner cylindrical body 4 can be placed to a position of a nearest one of a plurality of groove-like recessed parts provided to the inner circumferential face of an outer cylindrical body 1 in its axial direction. Then, a fixing bolt 16 is tightened and thereby the front ends of first and second pressing parts 15a and 15b of a pressing device 15 are brought into contact with the bottom of the groove-like recessed part through a notched hole provided to the inner cylindrical body 4. Then, turning of the inner cylindrical body 4 is regulated by friction force acting between the pressing device 15 and the bottom of the recessed part, and in the case where loosening occurs after a long time of use, the turning of the inner cylindrical body 4 can be checked with a groove-like stepped part provided to the recessed part. Thereby, occurrence of the loosening can be prevented effectively for a long period of time.