This paper demonstrates a novel and simple technique for the diagnosis of death by fire using a digital infrared (IR) camera system. At autopsy, the inhalation of soot by a fire victim is a definite indicator proving a vital reaction during the fire. However, there can be difficulties in confirming the presence of soot because of the relative lack of soot or the similarity in color between soot and surrounding tissues. To evaluate the effectiveness of IR imaging at autopsy, we acquired internal images of the respiratory and gastrointestinal system by both an ordinary color and IR cameras. Using our method, the inhaled soot is visible as black particles or deposits, while the blood is transparent and the surrounding tissue is whitened. This technique enables the detection of normally undiscriminating soot in an ordinary color image. This is the first report suggesting the usefulness of IR imaging in forensic autopsy for the diagnosis of death by fire.