Abstract: The present invention provides: a first sealing material for semiconductor device which sealing material is excellent in plasma resistance and inexpensive; and a second sealing material for semiconductor device and a method of manufacturing this sealing material wherein the sealing material has a good surface smoothness and a good dimensional precision. The first sealing material contains a fluororubber as a rubber component wherein the fluororubber inevitably contains a cured product of a fluorine-based elastic copolymer of a specific composition. The second sealing material is obtained by crosslinking, with ionizing radiation, a fluororubber preform containing a fluororubber component (a) (comprising a specific fluorine-based elastic copolymer) and a non-elastic fluororesin component (b) (comprising a vinylidene fluoride (co)polymer) in a specific ratio.