Provided are: method of producing a sulfur-containing proanthocyanidin oligomer by reducing the molecular weight of proanthocyanidin in plants so that they can be readily absorbed through the intestine of an organism; and a health food composition and a pharmaceutical composition which contain the resultant sulfur-containing proanthocyanidin oligomer as an active ingredient and which are useful for treating and preventing various lifestyle-related diseases and brain diseases caused by generation of active oxygen species as well as for preventing aging.