PROBLEM TO BE SOLVED: To provide a safe composition for inhibiting methane generation at the rumen of a ruminant, causing no damage to the natural balance of various kinds of microorganism groups that are symbiotic in the digestive system of the ruminant, having no anxiety of decrease in commercial value due to residue left in the body of a host animal. SOLUTION: The methane generation-inhibiting composition inhibits methane generation without making the rumen environment worse by giving at least one kind selected from lactobacillus derived from naturally fermented ewe milk (leaven), yeast, and oligosaccharide to a ruminant through oral administration wherein the methane generation-inhibiting effect is enhanced by additionally formulating nitrate therewith, and the lactobacillus and yeast comprise at least one kind of microorganism(s) belonging to Trichosporon, Candida, Leuconostoc, and Lactococcus, in particular, the oligosaccharide preferably comprises a galactooligosaccharide.
