Provided is a therapeutic agent for an influenza virus infectious disease, which utilizes an active ingredient derived from a natural product and has an excellent effect. The therapeutic agent for an influenza virus infectious disease includes, as an active ingredient, a β-glucan-containing composition obtained from a culture of a microorganism belonging to Aureobasidium sp. The therapeutic agent is capable of preventing an influenza virus infectious disease from becoming serious and is capable of promoting the healing of the influenza virus infectious disease.
