The present invention provides a receptor protein specifically recognizing bacterial DNA having an unmethylated CpG sequence, a genomic DNA encoding it, an experimental animal model useful for examining responsiveness of a host immune cell against a bacterial infectious disease. DNA encoding a receptor protein specifically recognizing bacterial DNA having an unmethylated CpG sequence is screened by BLAST search, a number of EST clones having high homology with various TLRs is screened, these clones are used as a probe to isolate a full-length cDNA from mouse macrophage cDNA library, and the sequence of bases of the cDNA is analyzed to confirm that it is TLR9 comprising a conserved regions such as LRR and TIR regions, and then a knockout mouse is produced to confirm that TLR9 is a receptor protein of oligonucleotides having an unmethylated CpG sequence of bacterial DNA.