Aims: Scedosporium apiospermum sometimes causes serious infectious diseases on the skin of immunodeficient subjects. Antifungal effects of fatty acid salts in soap against S.apiospermum were investigated under different water conditions. Methods and Results: Ultrapure soft water (UPSW) was generated by the water softener with cation-exchange resin. The calcium and magnesium ions were replaced with sodium ions in UPSW. Scedosporium apiospermum was incubated with different fatty acid salts that constituted soap in distilled water (DW), tap water (TW) and UPSW. After incubation, the number of fungi was counted. Among the fatty acids, palmitic acid salt (C16) reduced the number of S.apiospermum. UPSW enhanced the antifungal effect of C16 on S.apiospermum. The absence of both calcium and magnesium ions and the existence of sodium chloride in UPSW were responsible for its antifungal effect. In addition, repeated short-term treatment with UPSW and C16 decreased the number of S.apiospermum. Conclusions: Antifungal effects of C16 on S.apiospermum were demonstrated. Moreover, the use of UPSW promoted the antifungal effect of C16. Significance and Impact of Study: This study provides the preventive method for diseases associated with S.apiospermum infection using novel palmitic acid soap in UPSW.