A primer for amplifying a nucleic acid having a structure represented by formula (1): where A1 represents —S—, —S—S—, or —Se—, B represents a base, and R1 represents a decomposable protecting group, and the symbol * represents a bond to a sugar of an adjacent nucleotide. A device for producing double-stranded DNA includes: a forward primer and a reverse primer, having a structure represented by formula (1); a PCR device for forming double-stranded DNA with 3′-recessed ends by performing multiple cycles of PCR by using a template DNA as a template; Klenow fragment for making the 3′ ends blunt; and a photoirradiation unit for deprotecting R1 and forming a sticky end with a 3′-protruding end.