Disclosed is a means for improving the poor conversion efficiency in a conventional bioconversion system using a transformant which is given by introducing a gene originated from xerogenic organisms. A transformant is prepared by using a host which is defective in a gene encoding a multidrug efflux protein and introducing a gene originated from xerogenic organisms. Use of the transformant results in much effective microbial conversion of a hydrophobic or amphipathic substrate compound into a desired compound. In case, an Escherichia coli is used as the host, the gene encoding a multidrug efflux protein to be defective may be tolC, acrA, acrB and the like.