Soyasapogenol B is biosynthesized via two steps of hydroxylation reaction of its precursor β-amyrin. However, the gene of the hydroxylase concerned in this reaction has not been revealed. Therefore, it was impossible to apply a genetic engineering technique on the hydroxylase. The present inventors reveals that a sequence which corresponds to a soybean-derived cytochrome P-450 gene CYP93E1 encodes an enzyme protein that carries out hydroxylation of the 24-position of an oleanane type triterpene, and also provides a method for applying said gene making use of a genetic engineering technique.