In the present application are disclosed a method for enhancing or improving the flavor of foods or drinks in general with the use of a non-volatile thiazolidine compound alone or a non-volatile flavor compound and/or a reaction flavor concurrently with the non-volatile thiazolidine compound, a simple and effective method for improving the flavor of a retort food by suppressing the flavor-deterioration upon heat sterilization or the unpleasant odor at the time of eating, and a simple and effective method for improving the flavor of a soybean-incorporated food product by suppressing the unpleasant, weed-like odor peculiar to soybean.