Several 10-cm-long capillary tubes [made of poly(ether ether ketone) (PEEK)] with inside diameters of 0.1-2.0 mm were filled with silica monolith-immobilized protease derived by in situ sol-gel transition from a 1:4 mixture of tetramethoxysilane and methyltrimethoxysilane. Transesterification between 20 mM (S)-(-)-glycidol and 0.4 M vinyl n-butyrate in an organic solvent was used as the test reaction. The substrate solution flowed through the column at a flow rate of 0.0004-5.0 mL(.)min(-1). The conversion in the micro-bioreactor was higher than that in the batch reactor at a high liquid flow rate. When three tubes were connected in series, the conversion at a fixed ratio of the mass of the enzyme to the liquid flow rate was increased by approximately 50%, because of the tripling of the flow rate as compared to the case with a single tube. Changes in the tube diameter had no influence on the conversion at a fixed superficial liquid velocity. Further, the conversion increased with a decrease in the enzyme content. These results were ascribed to the apparent effect of liquid-solid mass transfer and were analyzed quantitatively using a simple mathematical model.