A method for crosslinking an isoprene-isobutylene rubber, an ethylene-propylene-diene rubber containing ethylidenenorbornene as an unsaturated component, or a mixture of an isoprene-isobutylene rubber and an ethylene-propylene-diene rubber containing ethylidenenorbornene as an unsaturated component, which method comprises adding, to an isoprene-isobutylene rubber, an ethylene-propylene-diene rubber containing ethylidenenorbornene as an unsaturated component, or a mixture of an isoprene-isobutylene rubber and an ethylene-propylene-diene rubber containing ethylidenenorbornene as an unsaturated component, an alkylphenol-formaldehyde resin, 3-(N-salicyloyl)amino-1,2,4-triazole and a hydrazide compound.