To replace only a failure part without replacing the whole flush valve body in a water cut-off failure caused by the corrosion and wear of a seat part of a piston valve.

The piston valve seat part of a flush valve body is provided with an internal thread, and a detachable valve seat is screwed to the internal thread. Only the screwed valve seat can thereby be replaced at the occurrence of defectiveness caused by the corrosion and wear of the valve seat part.