The present invention relates to a method for producing an oligonucleic acid compound, the method being characterized by (1) treating a compound represented by formula [A-1] and a compound represented by formula [B-1] with a condensation agent in the presence of a base and then (2) treating the compounds with an oxidizing agent and an organic amine to produce a compound represented by formula [C-1] (in the formulae, each symbol is as defined in the description).