Butterfly valve with flow straightening

A butterfly valve mountable in a valve body formed with a flow passage, the butterfly valve comprising:

• a valve shaft placed across the 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;
• wherein the valve element includes a front side and a back side each including at least a flow straightening surface so that a portion of the valve element including the flow straightening surface has a cross section having a thickness gradually decreasing from the valve shaft toward an outer edge in a direction nearly perpendicular to the valve shaft and being uniform in a direction parallel to the valve shaft,
• wherein the valve element includes a flow straightening rib on each of the front side and the back side and the flow straightening ribs thus provided are symmetric between the front side and the back side.