To provide a butterfly valve with a valve box seat cover, preventing damage to a valve box seat due to foreign matters in a fluid.

A valve element 3 to be rotated around the axial center of a valve rod 4 is arranged in a valve box 1. The valve box seat 1a in slide contact with a valve element seat 3a at the peripheral edge of the valve element 3 is arranged on the inner periphery face of the valve box 1. The valve box seat cover 6 of an elastic material for covering the valve box seat 1a is provided on the valve element 3 via a supporting member 5. In a fully open condition, the valve box seat 1a is covered with the valve box seat cover 6.