Disclosed are: a composition which can enhance the propionic acid bacterium proliferation activity in the rumen of a ruminant animal for the purpose of allowing the metabolic function of a propionic acid bacterium in the rumen of the ruminant animal, thereby preventing/treating rumen acidosis in the ruminant animal； and use of the composition. A bacterium capable of producing a propionic acid bacterium and a culture of a lactic acid bacterium are administered orally to a ruminant animal to dramatically (rapidly) enhance the proliferation of the propionic acid bacterium in the rumen of the ruminant animal and increase the concentration of a volatile fatty acid in the rumen. In this manner, rumen acidosis in the ruminant animal can be prevented and/or treated.
