The main irritant substance of Hatomugi, Coix Lacryma-Jobi Linné, var. Ma-yuen Stapf , was isolated. From the results of analysis of this substance by means of hydrolysis, thin layer chromatography, elementary analysis and infrared spectroscopy, the irritant substance in Hatomugi was presumed as a glucoside of 3, 4-dihydroxybenzaldehyde.