The aim of this study was to investigate the correlation between dying alone and suffering from a mental disorder based on forensic autopsy results over time. This study used results of forensic autopsies over 3 fiscal years based on a previous study, which used the same method. Among all the autopsy cases in 2016, 650 cases of people dying alone at their residences were identified. Of these, 181 suffered from a mental disorder. The tendencies pertaining to the cause of death of people suffering from mental disorders included high percentages of drug poisoning and suicide and were the same as those of the previous study. In addition, in comparison with people not suffering from mental disorders, the causes of death among those suffering from mental disorders included death because of reasons other than medical disease at a twofold higher percentage. We considered this result to strongly demonstrate that the presence of a mental disorder may be a major risk factor pertaining to people dying alone at their residences; therefore, steps for prevention of such cases are needed. Furthermore, people who have been identified to have died alone at their residences and have been suffering from mental disorders may have decreased ability of judgment and action in daily life.