To provide an offset butterfly valve having equal performance in both directions even when pressure is applied in any directions and preventing leakage.

When a valve seat 5 is provided in a butterfly 2 of this eccentric offset butterfly valve, the inner diameter of the valve seat 5 is set smaller than the bottom diameter of a groove 6 formed on an edge part of the butterfly 2 of the eccentric offset butterfly valve. When the valve seat 5 is provided in a valve element 1, the outer diameter of the valve seat 5 is set larger than the bottom diameter of a groove 6 formed on an edge part of the valve element 1 of the eccentric offset butterfly valve. The valve seat 5 can be easily installed, and has equal performance in both directions.