In a punching step, generation of flaws due to contact between an outer circumferential surface of a gasket and a punching blade, is reduced. A molded sheet forming step S1 of forming a molded sheet 8 in which rear end portions of a plurality of gaskets 1 aligned with each other are connected to each other via a joint portion 7, and a punching step S2 of punching the molded sheet 8 to separate each gasket 1 from the molded sheet 8, are provided. The punching step S2 includes: a positioning step S2A of positioning the molded sheet 8 by a positioning pin 12 being inserted into each plunger attaching hole 4 of the molded sheet 8 and a punching operation step S2B of separating each gasket 1 of the molded sheet 8 having been positioned, by using punching blades 13U and 13L.
