The present invention is to provide a β-1,4-mannobiose-containing composition which can inhibit colonization of salmonella in animal body and effectively excrete salmonella outside the body and the method for producing thereof, a feed additive containing the β-1,4-mannobiose-containing composition, and a feed blended with the β-1,4-mannobiose-containing composition. The present invention is intended to provide a method for producing the β-1,4-mannobiose-containing composition wherein a mannan degrading enzyme is functioned to a mannan-containing natural material for producing at least 10% by weight of β-1,4-mannobiose based on the mannan before the degradation, the β-1,4-mannobiose-containing composition prepared through the function of a mannan degrading enzyme to a mannan-containing natural material, which contains at least 10% by weight of β-1,4-mannobiose in terms of dry matter, a feed additive containing the β-1,4-mannobiose-containing composition, and a feed blended with the β-1,4-mannobiose-containing composition.