In order to utilize Apios americana flower as a functional food material, its safety and antihyperglycemic effects were investigated in mice. Normal mice fed dried A. americana flower or its methanolic extract (AFE) for 4 weeks did not show abnormalities in growth or plasma biochemistry profiles. However, ingestion of AFE lowered plasma glucose levels in ICR and diabetic KK-Ay mice. AFS60, fractionated from AFE by Sephadex LH-20 column chromatography, inhibited maltase; and suppressed the elevation of plasma glucose levels by 15% compared with control after oral administration of maltose in KK-Ay mice. Caffeoyl β-d-glucopyranoside was identified as the strongest maltase inhibitor in AFS60. While further toxicological studies are required, these results suggest that A. americana flower has maltase inhibitory activity and antihyperglycemic effects, and is a safe and useful novel food material for the prevention of diabetes.