PURPOSE: To effectively open and close the main valve body and the auxiliary valve body set in its central part of a butterfly valve by only one operation shaft through a gear mechanism.

CONSTITUTION: A main valve driving gear 4 and an auxiliary valve driving gear 6 are severally fittedly attached to a main valve shaft 8 and an auxiliary valve shaft 9 which have the same axial center, and a main valve driving pinion 3 fittedly attached to a rotary shaft 7 can engage intermittently with the main valve driving gear 4, and an auxiliary driving pinion 5 fittedly attached to the rotary shaft 7 can always engage with the auxiliary valve driving gear 6. A regulating ring 17 to adjust the swivel timing of the main and auxiliary valve bodys is insertedly attached between the auxiliary valve shaft and the auxiliary valve driving gear, and a segment-shaped teeth part is formed on the outer peripheries of the main valve driving gear and pinion, and moreover a broken tooth cylindrical part in which a nontooth cylindrical part can fit to selflock the main valve body is formed.