It is an objective of the present invention to develop a Grifola-derived substance having advanced immunopotentiating activity and antitumor activity, which has a molecular weight lower than that of a conventionally known glucan-protein complex and is structurally different from such complex. A Grifola-derived low-molecular-weight substance is produced by the following steps 1) to 4): 1) a step of thermally extracting mycelia or fruit bodies of Grifola with water； 2) a step of adding alcohol to the obtained water-soluble extract fraction to a final concentration between 10% and 80% by volume, allowing the resulting solution to stand at a temperature between 1°C and 40°C, and removing matter floating on or in the solution or adhering to the wall surface of a vessel and a precipitate formed therein； 3) a step of removing alcohol from the water-soluble fraction containing alcohol obtained in step 2) and collecting an adsorbed fraction from the obtained water-soluble fraction by anion-exchange column chromatography； 4) a step of collecting a fraction with a molecular weight of 5000 or less by subjecting the adsorbed fraction to molecular sieving.
