A process for producing a flavor-improving agent characterized by comprising heating yeast extract together with a sinigrin-containing material such as a plant belonging to the family Brassicaceae (for example, horseradish), a method of improving the flavor of a food or a drink by using this flavor-improving agent, and so on. The flavor-improving agent as described above is appropriately usable as an agent for imparting stewed meat flavor to a food or a drink prepared by stewing such as a sauce or a soup to thereby create a characteristic and rich flavor or a flavor with fullness obtained by stewing meat, for example, beef, pork, chicken or mutton, i.e., the stewed meat flavor.