We investigated the transgalactosylation reaction of chlorphenesin (CPN) using ${eta}$-galactosidase (${eta}$-gal)-containing Escherichia coli (E. coli) cells, in which galactose from lactose was transferred to CPN. The optimal CPN concentration for CPN galactoside (CPN-G) synthesis was observed at 40 mM under the conditions that lactose and ${eta}$-gal (as E. coli cells) were 400 g/l and 4.8 U/ml, respectively, and the pH and temperature were 7.0 and $40^{circ}C$, respectively. The time-course profile of CPN-G synthesis under these optimal conditions showed that CPN-G synthesis from 40 mM CPN reached a maximum of about 27 mM at 12 h. This value corresponded to an about 67% conversion of CPN to CPN-G, which was 4.47-5.36-fold higher than values in previous reports. In addition, we demonstrated by thin-layer chromatography to detect the sugar moiety that galactose was mainly transferred from lactose to CPN. Liquid chromatography-mass spectrometry revealed that CPN-G and CPN-GG (CPN galactoside, which accepted two galactose molecules) were definitively identified as the synthesized products using ${eta}$-gal-containing E. coli cells. In particular, because we did not use purified ${eta}$-gal, our ${eta}$-gal-containing E. coli cells might be practical and cost-effective for enzymatically synthesizing CPN-G. It is expected that the use of ${eta}$-gal-containing E. coli will be extended to galactose derivatization of other drugs to improve their functionality.