PROBLEM TO BE SOLVED: To provide a container-packed beverage containing milk or soybean milk capable of having fine foams, and a powdery foaming agent for generating fine foams in a container-packed beverage containing milk or soybean milk.SOLUTION: To a container-packed beverage containing milk or soybean milk, a powdery composition containing a carbonate and an organic acid is added after cap opening. The beverage preferably includes the powdery composition disposed not in contact with the liquid in the container, more preferably being stored at 35°C or lower, furthermore preferably containing a liquid acidulant in the liquid in the container. The liquid acidulant includes preferably at least one selected from the group consisting of gluconic acid, phytic acid, and phosphoric acid. A powdery foaming agent for the container-packed beverage containing milk or soybean milk includes a carbonate and an organic acid. The powdery foaming agent is added to a container-packed beverage, which is preferably not a hot beverage.
