Functional gastrointestinal disease is one in which gastrointestinal symptoms persist chronically or recurrently. This disease is challenging because it does not have an organic cause that can be detected in routine laboratory tests. Among them, the symptoms of irritable bowel syndrome (IBS), which is a type of functional gastrointestinal tract disease, include abnormal bowel movements associated with abdominal pain. However, no specific test has been established to definitively diagnose these diseases, including IBS. The traditional Rome IV diagnostic criteria are used to diagnose IBS by assessing subjective symptoms. However, it has been suggested that IBS is difficult to diagnose using the Rome IV criteria among unconscious or cognitively impaired patients. It is recommended that abdominal ultrasonography be used to assess IBS with diarrhea and constipation. Previously, constipation among elderly people who ingested food orally was objectively assessed by ultrasound, and colonic fecal distribution patterns were classified in constipated patients and healthy people. Objective visualization of the large intestine was used to assess constipation. Therefore, fecal retention among adults and elderly individuals was reported using ultrasonography. It was suggested that stool retention could be confirmed by observing the hyperechoic region of the rectum. Strong hyperechoic regions with acoustic shadows in the rectum indicate the presence of hard stools, thus enabling medical workers to identify constipation. In the future, ultrasonography may be useful for diagnosing IBS in unconscious patients or those with cognitive decline.