To obtain a feed composition for preventing infectious disease, free from adverse influence on the environment, having excellent safety to human body and effective for decreasing the death of edible animals such as fish, poultry and cattle by infectious diseases by formulating a betaine and dried beer yeast cell wall.

The objective feed composition is produced by formulating a betaine such as 1-carboxy-N,N,N-trimethylmethanaminium hydroxide separated e.g. from sugar syrup of beet belonging to the genus Beta and dried cell wall of beer yeast containing a large amount of glucan which is a kind of polysaccharides. The ratio of betaine to the dried beer yeast cell wall (betaine/ dried beer yeast cell wall) is 1/9 to 9/1 and the addition amount of the composition to a feed is preferably adjusted to give a daily administration rate of 0.5-0.8 g of the composition per 1 kg of the edible animal.