Primers for DNA elongation are immobilized onto a substrate having on the surface thereof a polymer substance which contains a first unit having a group derived from a phosphoester composing the hydrophilic section of a phospholipid, and a second unit having an activated ester group, and DNA elongation reaction is allowed to proceed in a reaction system having introduced therein a sample which contains DNA fragments having desired sequences and nucleotide monomers, by heating the reaction system up to a temperature causing thermal unfolding of the DNA chains, and then by cooling the reaction system down to a temperature for annealing.