PCR is effected by using chromosomal DNA of Corynebacterium thermoaminogenes as a template with plural primer pairs designed on the basis of domains conserved at the amino acid level among known gene sequences of various microorganisms corresponding to genes encoding L-amino acid biosynthesis system enzymes originating in Corynebacterium thermoaminogenes (preferably enzymes acting at higher temperatures than Corynebacterium glutamicum does). By using primers providing amplified fragments as screening primers, clones containing the target DNA fragments are selected from a Corynebacterium thermoaminogenes chromosomal DNA plasmid library.