Although it is recommended that the standard S-1 dosage should be based on how large the body surface area is, an on-site setting of the appropriate dosage is often lower than the standard one, depending on the individual's condition and considering possible side effects and so, on. Here, we investigated usage conditions for S-1 as a part of field training for expert pharmacists at our hospital that performs total clinical treatments. Decreases in dosage per day for elderly patients were although the standard dosage is generally determined according to the amount of a patient's body surface. We conducted a retrospective survey with a total 90 patients by creating a tree-diagram to identify a reduction standard. It was found that the S-1 dosage was decreased when there were side effects, aggravation in performance status, decrease in kidney function, old age, combined injection chemotherapy, and a decrease in radiation therapy performance. The dosage decreases without such medical reasons were seen in only 4 of the 90 patients. The individual target dosage on the basis of daily medical examination.