PURPOSE: To prevent a resin from flowing to adhere to a seat surface, by engaging a seat surface of a resilient seat with cores, forcing a projected section against the cores, placing reinforcing fibers in molds, and then injecting a resin under pressure.

CONSTITUTION: A resin valve box 1 is produced by a pair of cores 2 for forming its inner circumferential surface and the outer mold 3 for forming its outer circumferential surface. The seat surface of an annular resilient seat 4 of a soft rubber such as Cr, EPT, etc. is engaged with the cores 2, and the projected section 4a extending from the side wall is forced and secured to the inner surfaces of the cores 2 by an adhesive. Preformed reinforcing fibers 5 are placed in the molds 2, 3. A thermosetting resin such as an unsaturated resin or the like is injected via a gate 6 into the molds 2, 3, to mold a valve box 1 and the seat 4 integrally. A resin valve body 8 is associated to constitute a batterfly valve. Thus, since the seat 4 is engaged with the cores 2, the resin for the valve box can be prevented from flowing to the seat surface.