A method of determining a DNA tag which is a base sequence to be introduced into a genomic DNA sequence of an organism and an introduction site of the DNA tag into the genomic DNA sequence, the method comprising: a step S1 of obtaining a protein coding sequence from the genomic DNA sequence; a step S2 of determining a first coding sequence as a region to be treated from the protein coding sequence; a step S3 of obtaining a plurality of second coding sequences each including a partial sequence in the first coding sequence and having a predetermined length and recording sites of the second coding sequences in the protein coding sequence; a step S4 of obtaining one or more third coding sequences produced by subjecting the corresponding second coding sequences obtained in the step S3 to a silent mutation; a step S5 of judging whether or not the third coding sequences satisfy a predetermined condition and determining only the third coding sequences that satisfy the predetermined condition as fourth coding sequences; a step S6 of performing a homology search for the fourth coding sequences and obtaining homologous base sequences; a step S7 of determining an Nearest Match Score for each of the fourth coding sequences; and a step S8 of determining the fourth coding sequence corresponding to a minimum value of the Nearest Match Score as the DNA tag and determining the site of the second coding sequence corresponding to the fourth coding sequence as the introduction site, wherein: in the step 2, the first coding sequence comprises a coding sequence in a region having no influence on a biological function of the organism even if the sequence is subjected to the silent mutation; the predetermined condition comprises a condition where a polynucleotide including the third coding sequence or a complementary base sequence thereof is suitable as a primer; and the Nearest Match Score indicates a degree of homology. a step S1 of obtaining a protein coding sequence from the genomic DNA sequence; a step S2 of determining a first coding sequence as a region to be treated from the protein coding sequence; a step S3 of obtaining a plurality of second coding sequences each including a partial sequence in the first coding sequence and having a predetermined length and recording sites of the second coding sequences in the protein coding sequence; a step S4 of obtaining one or more third coding sequences produced by subjecting the corresponding second coding sequences obtained in the step S3 to a silent mutation; a step S5 of judging whether or not the third coding sequences satisfy a predetermined condition and determining only the third coding sequences that satisfy the predetermined condition as fourth coding sequences; a step S6 of performing a homology search for the fourth coding sequences and obtaining homologous base sequences; a step S7 of determining an Nearest Match Score for each of the fourth coding sequences; and a step S8 of determining the fourth coding sequence corresponding to a minimum value of the Nearest Match Score as the DNA tag and determining the site of the second coding sequence corresponding to the fourth coding sequence as the introduction site, wherein: in the step 2, the first coding sequence comprises a coding sequence in a region having no influence on a biological function of the organism even if the sequence is subjected to the silent mutation; the predetermined condition comprises a condition where a polynucleotide including the third coding sequence or a complementary base sequence thereof is suitable as a primer; and the Nearest Match Score indicates a degree of homology.