The present invention provides a hemiacetal ester prodrug of the pyridone carboxylic acid antibacterial drug, wherein the prodrug is characterized in that, (i) it is hydrolyzed promptly after intestinal absorption and regenerates the pyridone carboxylic acid antibacterial drug as a drug substance, (ii) a one-dose package is possible by preventing chelate-formation in intestines in an oral administration, and (iii) the normal bacterial flora of the intestinal bacterial flora is not influenced at all until the ester is absorbed, because the hemiacetal-type esterification is executed on the carboxylic acid which is a center of the antibacterial action, resulting in prevention of the pseudomembranous colitis. The prodrug is an alkoxy carbonyl hemiacetal-type ester represented by the chemical formula (I), which is useful as a prodrug for inhibiting the enteric-bacterium-shift side effect, and can be used in combination with polyvalent metal-containing medicines.
