A plastic butterfly valve includes a hollow cylindrical body in which an annular seat is fixed on the inner peripheral surface of a flow passage of the body by a seat pressure foot, and a valve disk rotatably and eccentrically supported at the stem of the valve disk within the body to abut the periphery of the valve disk on the seat. The seat pressure foot is detachably fitted in one side portion of the body by an insertion/rotation mechanism. The insertion/rotation mechanism includes a plurality of circular arc projections on the outer peripheral end surface of the seat pressure foot, arcuate cutout portions arranged at positions corresponding to the projections in the body, and an engaging groove on the interior sides of the cutout groove on the interior sides of the cutout portions to guide the projections. By using the insertion/rotation mechanism, mounting and demounting the seat on and from the body can be easily carried out and assembling of the seat and the body can be carried out in a short time.