IPTG induction caused a sudden drop of alkali consumption rate during cultivation of a recombinant E. coli with ${eta}$-galactosidase structural gene under T7 promoter on a plasmid. A series of batch cultivations showed the positive correlation of the decrease of alkali consumption and the level of expression. However, repeated IPTG induction did not cause any variation of alkali consumption rate. Supplementation of medium even at stationary phase enhanced the level of ${eta}$-galactosidase expression. These results suggests that the drop of alkali consumption rate by IPTG induction represents the rate of expression.