Following preparation of water extracts of Pleurotuscornucopiae fruiting body-containing α-glucosidase inhibitor, their antihyperglycemic effects were examined using streptozotocin-induced diabetic Sprague-Dawley (SD)-rats. The water extracts from Pleurotuscornucopiae showed dosage-dependant antihyperglycemic effects on the streptozotocin-induced diabetic SD-rats after oral administration to 120 min on the short time test and 4 days on the long time test, respectively. The water extracts from Pleurotuscornucopiae fruiting body showed dosage-dependent hypoglycemic action after administration to 120 min and 4 days in the SD-rat and streptozotocin-induced diabetic SD-rat.