The object of the invention is to enable production of high quality eggs in poultry and to provide a method for improving productivity thereof. The invention provides a method for producing a hen egg having improved quality and productivity, the method characterized in having a step for feeding a hen on supplemented feed obtained by adding to poultry feed an immunostimulating substance produced by cytolysis that accompanies sporulation of MRE symbiotic bacteria and/or on 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 symbiotic bacteria from the culture fluid.