A nucleotide chain to be modified, a nucleotide having a particular base that is different from bases constituting the nucleotide chain, an enzyme catalyzing addition of the nucleotide to the 3′-terminus of the nucleotide chain, a degrading enzyme acting specifically on the nucleotide, and a desired modifier for modifying the nucleotide chain are allowed to coexist in a buffer solution as a mixture solution such that: the nucleotide is added to the 3′-terminus of the nucleotide chain; the sequence of the added nucleotide is degraded to form, at the 3′-terminus of the nucleotide chain, a functional group capable of binding to the modifier; and the 3′-terminus of the nucleotide chain having the functional group thus formed is directly modified with the modifier. The reactions at three stages continuously proceed in the mixture solution. As a result, simplified procedures and reduced reaction time can be achieved.