A casing of a fixed disc memory unit consisting of a case body and a covering member jointly forming the casing with an air-tight sealing of a rubbery gasket therebetween can be imparted with very reliable moisture-proofness of sealing by forming the rubbery gasket by the FIPG method using a curable rubber composition comprising: (a) a fluorocarbon-containing organosiloxane polymer containing vinyl groups and consisting of the recurring units each represented by the general formula--[--SiMe.sub.2 --CH.sub.2 CH.sub.2 --Rf--CH.sub.2 CH.sub.2 --SiMe.sub.2 --O--]--,in which Rf is a fluorinated divalent polyether group; (b) an organohydrogenpolysiloxane or a fluorocarbon-containing organohydrogensiloxane having, in a molecule, at least two hydrogen atoms directly bonded to the silicon atoms; and (c) a catalytic compound in an amount sufficient to effect the addition reaction between the components (a) and (b).