PROBLEM TO BE SOLVED: To provide a preparation process capable of extracting aβ-D-glucan contained in Agaricus Blazei Murill into a liquid in the quantity more than that by conventional preparative processes. SOLUTION: The preparation process comprises the steps of pulverizing Agaricus Blazei Murill into microparticles of from 20μm to 1μm with a pulverizer, pressing them, and extracting aβ-D-glucan into a liquid by means of a hot water extraction method.