A simple method for measurement of α-amylase activity in rice koji with 2-chloro-4-nitrophenyl 6⁵-azido-6⁵-deoxy-β-maltopentaoside (N3-G5-β-CNP) as a substrate was developed, because methods using soluble starch have been somewhat complicated, especially for assays of a large number of koji sarnples.In the proposed method, 2-chloro-4-nitrophenyl β-maltosaccharides released from the substrate on action of α-amylase was immediately cleaved to 2-chloro-4-nitrophenol (CNP) in acoupled reaction catalyzed by glucoamnylase and β-glucosidase. The increment of CNP, measured at 400nm, was directly proportional to the amount of α-amylase present.The maximum activity of α-amylase from Aspergillus oryzae was found around pH 4.2 in this study.The average recovery of α-amylase added to rice koji extracts was 99.6%. Coeff icients of variation (CVs) in within-day and day-to-day experiments were less than 1% and less than 2%, respectively.This method was not affected by glucoamylase and glucose present in rice koji extracts. The values of α-amylase activities measured by the proposed method correlated well with those by a generally used method (National Tax Administration Agency). It would fulfill the need for anaccurate, specific and simple assay of α-amylase.