Abdominal pain and marked coldness were observed in a 56-year-old male with cancerous ascites associated with cancer of unknown primary, and the patient was treated with sekiganryo. The abdominal pain resolved from day 2 after the initiation of sekiganryo administration, and the ascites markedly decreased with an increase in the urine volume on day 3. Although the mechanism of the effect of sekiganryo on cancerous ascites was unclear, it was considered that the improvement of coldness promoted the metabolism and subsequently promoted diuresis. No case of the improvement of cancerous ascites by sekiganryo has been reported to date. We report this case along with a literature review.