Provided is a method for selectively obtaining, for a given target gene, a “joined DNA fragment” wherein just a target gene fragment is joined with desired other DNA fragments, regardless of whether a DNA fragment containing a target gene sequence has been purified. In the provided method, a double-stranded joining DNA fragment containing a sequence A and/or a sequence B is selectively joined to the ends of a target gene fragment. A mixture of a double-stranded gene fragment, the 3′ end of which is protruding, and the double-stranded joining DNA fragment, which are related in a prescribed manner, undergoes at least two cycles of thermal denaturation, reassociation, and DNA synthesis, resulting in a “joined DNA fragment,” which is a double-stranded DNA fragment including at least one instance of a sequence resulting from joining sequence A, the target gene sequence, and sequence B. A “single-side joined DNA fragment” can also be obtained, by a similar method.