To provide a method of remodeling a tank close-fitted flush toilet seat capable of improving the presumable defects caused thereby when constructing a reclaimed water utilization system by partly remodeling a clean water utilization system.

In this flush toilet seat remodeling method for remodeling the tank close-fitted type flush toilet seat having a feed water tank closely fitted to a water closet body and utilizing clean water into the close-fitted type water closet utilizing reclaimed water, the internal hardware of the feed water tank is replaced with the internal hardware suitable for utilization of reclaimed water.