PURPOSE: To achieve the simplification of work and the reduction in the man hour by cooling a valve seat having a desired shrinkage allowance and interference to reduce the smae to the size at which it can be inserted or screwed into a valve seat mounting hole then tightly fitting this valve seat into the mounting hole of the valve box or valve body by causing the expansion restorage of the valve seat.

CONSTITUTION: A valve seat 1 composed of blank of bronze or the like is so worked as to have a desired shrinkage allowance and interference and is so worked as to permit its insertion or screwing into a valve seat mounting hole 4. Next, the valve seat 1 is cooled preferably to about -70°C so that it is shrank to the size at which it can be inserted or screwed into the mounting hole 4. Thence, this valve seat 1 is inserted or screwed into the valve seat mounting hole 4 of the valve box 2 or valve body 3. The valve seat 1 mounted expands as it restores ordinary temperature and the outside diameter of the valve seat restores the original size and therefore the fitting part 1a of the valve seat 1 tightly fits into the mounting hole 4 owing to its desired interference, and the contact part 1b tightly fits to the contact surface 2b of the valve box 2 or the contact surface 3b of the valve body 3.