The present invention relates to a water-insoluble glucan purification method characterized in that a culture solution of a microbe that produces water-insoluble glucan or a microbial cell-containing solution comprising water-insoluble glucan as a component is treated with both an oxide of 0.175-3.5 wt.--wt. % and a hydroxide such that the pH value becomes 10-12.5. Since the present invention accomplishes all of the destruction of production microbe, the decolorization and the reduction of viscosity of a culture solution of a microbe that produces water-insoluble glucan or a microbe cell-containing solution comprising water-insoluble glucan as a component, the invention purifies water-insoluble glucan industrially at low costs and with a high efficiency.