The present invention provides a method for crosslinking an isoprene-isobutylene rubber, which comprises adding, to 100 parts by weight of an isoprene-isobutylene rubber, 5 to 25 parts by weight of an alkylphenol-formaldehyde resin and 0.1 to 5 parts by weight of a hydrazide compound, or 5 to 25 parts by weight of an alkylphenol-formaldehyde resin, 0.1 to 5 parts by weight of a hydrazide compound and 0.3 to 10 parts by weight of an epoxy compound. According to this method, a crosslinked rubber product of high hardness can be obtained without using any halogen compound or increasing the amount of carbon black used.
