A 57-year-old male patient with >10-year history of type 2 diabetes presented with a left big toenail deformity and pain. A physical examination revealed a white and yellow-to-brown patch on the nail as well as thickening and ingrowth of the nail plate. The nail plate was opened using nippers, and a fungal culture revealed Trichophyton interdigitale with yellow yeast. The yeast isolate was identified as Kocuria koreensis, a Gram-positive aerobic coccoid with keratinolytic properties that is part of the normal flora of the skin. We created an ex vivo onychomycosis model of T. interdigitale infection of the human nail by placing a sterilized normal nail on the cultured slant. K. koreensis initially spread over the normal nail, and T. interdigitale then penetrated the nail plate. After one year and six months, a spiral ingrown nail developed. A histopathological examination of the spiral revealed onychomycosis with superficial and deep abscesses of Gram-positive cocci infection. We performed PCR from paraffin-embedded material, and the sequences obtained were identical to those of T. interdigitale and K. koreensis. These results suggest that the development of onychomycosis by T. interdigitale is introduced and accelerated by K. koreensis, and the symbiosis of these microorganisms is suspected in the nail. This ex vivo model has a number of limitations. Therefore, further research on co-infected cases is needed to confirm this hypothesis.