We report two cases of irritable bowel syndrome (IBS) with predominant constipation successfully treated with daijokito and then diagnosed as Alzheimer's dementia. [Case 1] An 82-year-old man. He had been treated for IBS with probiotic and purgative. However, the symptom of IBS did not get cured and he had suffered from digestive symptom with iracund mental condition. [Case 2] A 74-year-old man. He suffered from abdominal pain and constipation so he needed enema and stool extraction frequently. He sometimes angered if he could not get treatment without delay. In both cases, their bowel and mental condition improved after treatment with daijokito. After their conditions improved, we checked their cognitive function, and found the levels of their cognitive function were low. We diagnosed them as Alzheimer's dementia. The patients probably had felt the difficulty and gotten mental stress on usual days because of dementia, so the mental stress could complicate the symptoms of IBS. Daijokito probably improved the digestive symptom due to IBS and the mental stress from dementia of these patients, and after the treatment we could diagnose the dementia. Elderly patients with constipation and iracund mental condition might have dementia.