The present invention provides a process for producing a wort with reduced content of a stale flavor-causing substance for the production of fermented malt beverages such as beer and the like in which generation of a stale flavor is suppressed, and it also provides fermented malt beverages in which generation of a stale flavor is suppressed such as beer and the like, that are produced by using wort with low content of a stale flavor-causing substance obtained by this process for producing. The second wort with reduced content of a stale flavor-causing substance is obtained with trans-2-nonenal potential (NP) as an indicator in the wort separation of a prepared wort in the production of fermented malt beverages. Fermented malt beverages with suppressed generation of a stale flavor-causing substance is produced by using wort in which NP in the whole wort is reduced by at least 1 ppb in terms of sugar content 11.degree. p.