A butterfly valve comprises a valve shaft placed across a flow passage and a plate-like valve element provided on the valve shaft so as to be rotatable about the valve shaft to regulate a flow rate of fluid in the flow passage. The valve element has a section that gradually decreases in thickness from the valve shaft toward an outer edge in a direction nearly perpendicular to the valve shaft. The valve element further includes a plurality of inclined surfaces on each side so that the corresponding inclined surfaces on both sides of the valve element are identical in section in a direction parallel to the valve shaft. The valve element is provided on each side with a plurality of flow straightening ribs each extending in a direction perpendicular to the valve shaft and inclining along each inclined surface.