[Problem] To enable production of high quality eggs in poultry, and to provide a method for improving production thereof. [Solution] A method for producing a hen egg having improved quality and productivity, the method characterized in comprising a step for feeding to a hen a supplemented feed obtained by adding to poultry feed an immunostimulating substance produced by cytolysis that accompanies sporulation of MRE symbiotic bacteria and/or by feeding supplemented drinking water obtained by adding the immunostimulating substance to drinking water； wherein the immunostimulating substance is obtained by culturing the MRE symbiotic bacteria, leaving the resulting culture fluid in a depleted state to thereby cause the symbiotic bacteria to convert to endospores, and removing impurities, including the endosporic bacteria, from the culture solution.
