Malignant melanoma occurring in burn scars is rare and cases of malignant melanoma and squamous cell carcinoma arising in a burn scar are extremely rare. We report a case of malignant melanoma and squamous cell carcinoma arising in one tumour on a stable thermal burn scar on the right leg of a 55-year-old man after a long latent period of about 50 years. The case was unique in that the malignant melanoma and squamous cell carcinoma occurred synchronously next to each other and produced one tumour. Immunohistochemical stainings with keratin, S-100 protein and HMB 45 clearly distinguished the two kinds of atypical tumour cells. Following the total resection of the original tumour, metastasis of malignant melanoma in the inguinal lymph node was found. This case underlines the possibility that another tumour may co-exist even if pathological observation reveals one kind of tumour.