as used herein u201c corresponding to both terminal sequences u201d means that each ssdna includes in any region 5 u2032 terminal or 3 u2032 terminal sequence in the second dsdna or a nucleotide sequence substantially identical to portions thereof the term u201c having a nucleotide sequence substantially identical u201d means as described above having the sequence equivalent enough for allowing the formation of triplexed dna by reca protein then in the third step the reca protein and the second ssdna are added to the dna library as described above this step causes the progress of triplexed dna formation reaction next in the fourth step dnas containing the triplexed dna obtained in the previous step are ligated with a desired dna while ligation does not occur on triplexed dna ligation occurs on untriplexed dsdna giving circular dna in this method u201c a desired dna u201d generally although not necessarily means a vector for constructing a library like phages cosmids and plasmids in the fifth step removal of linear dna from the dnas prepared in the previous step constructs a dna library having increased proportion of the first dsdna linear triplexed dna to be removed can be separated from circular dna to be increased in proportion as described above specific reactions and structures illustrated in the drawings are intended merely to help understanding accordingly their details may not be consistent with those shown in figures the invention is further illustrated in more detail by the following examples in this example we removed one plasmid from a mixture of two plasmids by ligation inhibition reaction using reca protein we used a chloramphenicol resistant plasmid pbc23c10 and an ampicillin resistant plasmid pbs1b3 and removed one plasmid from the mixture of the two plasmids pbc23c10 and pbs1b3 were mixed.