There is provided a preparation process useful for an efficient synthesis of 6-O-substituted ketolide derivatives by combining a characterized step of introduction of a substituent at the 6-position by selective cleavage of a C—O bond of the cyclic acetal at the 9-position side via 6,9-cyclic acetal 5-O-desosaminyl erythronolide derivative, a step of conversion into carbonyl groups at the 9- and 3-positions, and a step of 11,12-cyclic carbamation to lead to 6-O-substituted ketolide derivatives.