A plurality of primer sets are designed based on a region where conservation at the amino acid level is observed among various microorganisms for known gene sequences corresponding to a gene coding for an enzyme of the L-amino acid biosynthetic pathway derived from Corynebacterium thermoaminogenes, preferably an enzyme that functions at a higher temperature compared with that of Corynebacterium glutamicum. PCR is performed by using the primers and chromosomal DNA of Corynebacterium thermoaminogenes as a template. The primers with which an amplification fragment has been obtained are used as primers for screening to select a clone containing a target DNA fragment from a plasmid library of chromosomal DNA of Corynebacterium thermoaminogenes.