It is an objective of the present invention to develop a Grifola-derived glucan-protein complex that has a molecular weight lower than that of a conventionally known high-molecular-weight glucan-protein complex and has advanced immunopotentiating activity and antitumor activity. The glucan-protein complex of interest was obtained through the following 1) to 5): 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 20% and 70% by volume, allowing the resulting solution to stand at a temperature between 1° C. and 25° C., and collecting matter floating on or in the solution or adhering to the wall surface of a vessel; 3) a step of dissolving the collected matter in water and collecting non-adsorbed fraction by anion-exchange column chromatography; 4) a step of adding alcohol to the resulting solution to a final concentration between 20% and 50% by volume, allowing the solution to stand at a temperature between 1° C and 25° C., and removing the precipitate that had formed therein; and 5) a step of adding alcohol to the resulting solution to a final concentration between 40% to 99% by volume, allowing the resultant to stand at a temperature between 1° C. to 25° C., and collecting the precipitate that had formed therein.