The case was a man in his 80s, who had been in home care with primary hepatocellular carcinoma before hospitalization. He suffered nausea and appetite loss. However, hypercalcemia in laboratory data, brain metastasis on head CT and MRI, and intestinal obstruction or dilatation on abdominal CT, were not revealed. Upper gastrointestinal endoscopy detected no mechanical abnormality after admission to hospital and medical treatment with 3.75mg/day of mirtazapine was started. The dietary intake increased on the following day of the administration and nausea and appetite loss were improved, so the patient was discharged. In conclusion, low-dose mirtazapine was considered to be one of the effective therapeutic agents for nausea from unknown causes in cancer patients.