Plant expression vectors comprising an ORF of an isoform gene (.beta.1,4GT5) of human .beta.1,4-galactosyltransferase were constructed. Then, the vectors were introduced into tobacco using plant gene-recombination techniques, and transformed tobacco was made. Analysis of the obtained transformed tobacco showed that more than 200 .mu.g of lactosylceramide was produced from 1 g of fresh leaves. Accordingly, the present invention made it possible to mass synthesize lactosylceramide by using recombinant plants.