The popularity of keeping stag beetles (Dorcus titanus castanicolor Motschulsky 1861, Coleoptera: Lucanidae) as pets has increased. Consistent with the rise in the number of insect farms using these beetles, the number of contaminated or diseased D. titanus castanicolor has also increased. This investigation was conducted to analyze the cause of D. titanus castanicolor disease. The contaminated larvae of D. titanus castanicolor showed Allomyrina nudivirus infection symptoms similar to those of Allomyrina nudivirus infection. However, the disease carried by of D. titanus castanicolor is not derived from the virus infecting Allomyrina, as determined by PCR. Our study revealed that the major gut microbes of infectious D. titanus castanicolor belonged to the phylum Proteobacteria, and specifically, Pseudomonas knackmussi (Symptom 1 - 39.62% to Symptom 2 - 41.50% to Symptom 3 - 76.76% as the disease progressed severely) and Citrobacter koseri (Symptom 1 - 1.48% to Symptom 2 - 6.04% to Symptom 3 - 6.16% as the disease progressed severely) were detected. Additionally, a high proportion of larvae from the uninfected group were found to harbor bacteria belonging to the phylum Firmicutes (72%). However, as the disease progressed severely in these beetles, the proportion of Firmicutes decreased (Symptom 1 - 72.03% to Symptom 2 - 44.7% to Symptom 3 - 26.3%). These findings imply that colonization by Firmicutes was inversely proportional to Proteobacteria colonization in the gut. This was found to be true for both the normal and disease conditions of D. titanus castanicolor. In this study, we examined the distribution of intestinal microbial communities in normal and contaminated larvae. We observed a correlation between these contaminated microbes and the overall health of the beetle, and our findings suggest that there may be a link between disease progression and the gut microbiome.