Abstract: An object of the present invention is to provide an efficient method for producing a nucleic acid oligomer. An object of the present invention is to provide a method for producing a nucleic acid oligomer represented by formula (2) (in the formula, the symbols have the meanings described in the specification), including a step of reacting a nucleic acid oligomer represented by formula (1) (in the formula, the symbols have the meanings described in the specification) with a trichloroacetic acid solution in which a molar ratio of formaldehyde to trichloroacetic acid (formaldehyde mol/trichloroacetic acid mol) is 11×10?5 or less.

Abstract: The present invention provides a method for efficiently producing a nucleic acid oligomer. Specifically, the present invention provides a method for producing a nucleic acid oligomer represented by Formula (2), the method including a step of reacting a nucleic acid oligomer represented by Formula (1) with a dichloroacetic acid solution having a molar ratio of formaldehyde to dichloroacetic acid (formaldehyde mol/dichloroacetic acid mol) of 90×10?6 or less: wherein symbols have the meanings given in the description; wherein symbols have the meanings given in the description.