The cause of bitterness generated by the buckwheat shochu and its solution were investigated. The bitterness of the buckwheat shochu was not generated by phenolic compound, etc., but by excessive isoamyl alcohol and little n-propyl alcohol. This might have been caused by the water-soluble protein in the buckwheat seed. There was an isoelectric point near 8 on this water-soluble protein, which affected the flavor component generation and amino acid metabolism of the yeast. This water-solubleprotein was decomposed by the protease, and its influence to the yeast was abolished. It seemed possible that the isoamyl alcohol quantity could decrease and that there -would be littlebitterness when the added enzyme was deactivated with heat after the acid protease was made to work with the raw material buckwheat.