Legionella pneumonia is mainly caused by the inhalation of Legionella contaminated aerosol, aspiration or wound infection. Patients undergoing a severe immunosuppressive therapy such as organ transplant or cancer therapy are particularly at risk of infection. This study reported a case of Legionella pneumonia caused by Legionella pneumophila in a man with malignant melanoma. The method for diagnosing Legionella pneumonia by standard culture method was elaborate. A rapid diagnostic method derived from a two-step scheme assay with 99.42% accuracy for identification and differentiation of L. pneumophila and non-L. pneumophila in environmental Legionella isolates were also utilized in this study to evaluate whether this method was available for diagnosing Legionella infection. To confirm the diagnosis result of this rapid diagnostic method, sequencing of the bacteria in patient's sputum partial gene was also carried out. The diagnosis result of this rapid diagnostic method was consistent with the culture method which indicated that it was effective in diagnosing L. pneumophila infection.