The present invention discloses a sulfur-containing phenolic resin which comprises an organic group represented by the following formula (1): --R.sup.1--S--R.sup.2--S--R.sup.1-- (1) wherein R.sup.1 represents a hydrocarbon group having 2 to 6 carbon atoms, R.sup.2 represents a hydrocarbon group having 1 to 10 carbon atoms, between a phenolic carbon and a phenolic carbon, phenol derivatives represented by the following formula (5): ##STR00001## wherein R.sup.5 represents a C.sub.2-3 alkylene group, R.sup.6 represents a C.sub.1-10 alkylene group, G represents H, a C.sub.1-10 alkyl group, etc., and an epoxy resin composition containing (A) a curing agent of the above-mentioned formula (5) and (B) an epoxy resin as essential components.