Provided is a convenient, efficient, cost-effective method for introducing 1,4,7,10-tetraazacyclododecane-N,N,N",N"-tetraacetic acid (DOTA) into a compound such as a peptide.A method of introducing DOTA into a compound, comprising a first step for preparing a mixed liquid of DOTA having no protecting group and dimethylsulfoxide, and a second step for contacting the mixed liquid with the compound carried on a solid-phase carrier.
