A process for preparing S type 2-substituted hydroxy-2-indolidinylbutyric ester compound [II]: wherein Ro is residue of nitrogen-containing fused heterocyclic carboxylic acid having absolute configuration of “R” (in which the nitrogen atom is protected), R1 and R2 are lower alkyl groups, and E is ester residue, which is useful as an intermediate for preparing camptothecin derivatives having antitumor activities, which comprises 2-ethylating 2-substituted hydroxy-2-indolidinylacetic ester compound [I]: wherein the symbols are as defined above.