A rubber composition containing 100 parts by weight a sulfur-vulcanizable rubber (A) and 0.1 to 30 parts by weight of a vulcanization agent of a cyclic polysulfide (B) having the formula (I): ##STR00001## wherein R is a substituted or unsubstituted C.sub.2 to C.sub.20 alkylene group, substituted or unsubstituted C.sub.2 to C.sub.20 oxyalkylene group or alkylene group having an aromatic ring, n is an integer of 1 to 20, x is a number of 2 to 6 on average.