A method has the steps of forming a lot of potting portions made of a rubber-like elastic material at appropriate intervals in respect of an extending direction of the gasket main body in the one side in the thickness direction in the peripheral edge portion of the plate in such a manner as to be positioned in a forming area of the gasket main body, next, pinching a portion constructed by the peripheral edge portion of the plate and the potting portion between inner surfaces of a gasket forming cavity of the metal mold and bringing the other side in the thickness direction of the peripheral edge portion into close contact with the one inner surface of the cavity by positioning and arranging the plate within the metal mold and clamping the mold, and filling a liquid rubber material into the cavity and curing the liquid rubber material.