We presented an unusual case of negligent homicide by thorax compression, which is the expanded concept of traumatic asphyxia. A 58-year-old man was restrained in the prone position by six prison officers. They were ordered by their superiors to continue restraining him for about 15 min and the victim died. At the forensic autopsy, typical findings of thorax compression with intramuscular hemorrhages on the back and multiple fractures of the ribs were observed. No evidence of neck compression/smothering or other fatal issues likely to occur by chest compression was found. The reconstruction of the scene corresponded exactly with the localization of the injuries found in the victim. This is the first case of death by pure thorax compression without other fatal factors during intentional restraint, in which the force causing the chest compression was distinctly determined by the autopsy and reconstruction.