To keep high durability, quickly implement a preliminary servicing work at an original place without demounting a butterfly valve, and easily service the butterfly valve.

This valve has a body 2 with a circular passage, a butterfly disc 3 symmetrical about a median face and rotatably fitted to the body 2 for opening and closing the passage, shafts 7 and 8 for rotatably fitting the butterfly disc 3 to the body 2, and an annular seal seat 1 between the body 2 and the butterfly disc 3. Furthermore, the shafts 7 and 8 extend to the median face of the butterfly disc 3 and two holes 5 and 6 are drilled through the seal seat 1 for passing the shafts 7 and 8. The seal seat 1 is made of hard and rigid material, and a part of the internal lateral side thereof for contact with the butterfly disc 3 at a close position is formed to be spherical. Also, a part of the external lateral side of the butterfly disc 3 for contact with the seal seat 1 is formed to be spherical and the external lateral side of the seal seat 1 is kept away from the body 2.