PURPOSE: To provide high growth promoting action and efficiently culture a plant tissue on an industrial level without relying on plant seeds by adding a growth promoter containing an oligosaccharide having the growth promoting action for culturing the plant tissue into a culture medium in culturing the plant tissue.

CONSTITUTION: Soybean protein whey is diluted with water and heat-treated and a solution of calcium chloride is then added thereto. The pH of the resultant solution is subsequently regulated to 4.8-5.0 and the regulated solution is allowed to stand for 16hr to collect the clear supernatant solution. The obtained clear solution is treated with an ultrafiltration membrane and the treated solution is electrodialyzed with an ion exchange membrane, further passed through an ion exchange resin column and then concentrated to prepare an oligosaccharide syrup. A growth promoter containing the oligosaccharide having growth promoting action on a plant tissue for culturing the plant tissue is provided therefrom and the resultant growth promoter is added into a culture medium to carry out culturing of the plant tissue (e.g. a secondary adventitious bud of Quercus acutissima Carr.). Thereby, the plant tissue is efficiently cultured on an industrial level.