Abstract: A butterfly valve is described, including: a valve body made from synthetic material and having an internal cylindrical fluid passage; a movable disk valve disposed in the valve body for controlling the opening and closing of the fluid passage; a seated ring made from elastic material inserted between the valve body and the disk valve for closing the fluid passage when the disk valve is in a closed position; a shaft hole formed in the valve body into which a stem is inserted; a shaft cylinder formed to extend outwardly from an outer circumference of the valve body in corresponding with the shaft hole, the stem being inserted into the shaft cylinder and holding the valve axially, the valve body being divided along at least one division line formed across the fluid passage and being composed of plural divided parts, each divided part being joined at a joining surface extending circumferentially to form one complete valve body and being produced with a plurality of cavity portions in order to achieve light weigh