There are many sudden unexpected infant death cases which are easily diagnosed as sudden infant death syndrome (SIDS) both with and without autopsy in Japan. A SIDS diagnosis may offer a cover for accidental or criminal death. SIDS should not become a convenient diagnostic box that shelters the cases of unexpected infant death lacking the necessary antemortem information to make the correct diagnosis. The authors consider that SIDS should be diagnosed according to the direction of the international definition of SIDS, and proposed the following essentials for a forensic pathological diagnosis: (1) a thorough autopsy should be performed based on precise autopsy protocol, including a historical observation and toxicological, bacteriological, viral and/or biochemical examinations; (2) the forensic pathologist should be provided with pertinent information regarding antemortem health status, past clinical history, social circumstances, death scene investigation, etc., in order to collect more precise information, the authors recommended using a questionnaire to record information from the deceased’s guardians; and (3) suspicion of accidental death or infanticide should be completely ruled out. SIDS should be diagnosed only after these three essentials have been satisfied, if there is even a slight suspicion of accidental death or infanticide, or when pertinent information about the deceased, the cause of death should be diagnosed as unspecified or undetermined. This means that the causes and classification of the death are undetermined as to whether it is a natural or unnatural death. It is emphasized that the physician has the responsibility to report an infant found dead or dying of unnatural or clinically unexplained causes be reported to the police. This is considered the crucial first step in getting an accurate diagnosis of SIDS.