An object of the present invention is to increase a protein content in milk of a lactating cow. When 5-ALA, an amino acid, or ALAs which are derivatives thereof or salts thereof, are mixed with a feed and administered to a lactating cow without being treated such as coating, a lactation yield remained substantially unchanged but the obtained milk had significantly high proportion (%) of proteins, particularly casein, when compared with milk collected from milk cows who had consumed a conventional feed.
