The present invention relates to the soluble glucan oligomer having a M.W. ranging from 1,000 to 10,000 prepared by treating insoluble beta-glucan isolated the cell wall of yeast variant IS2 with commercially available beta-glucan hydrolyzing enzymes. The soluble glucan oligomer showed potent efficacy on inhibiting activity of influenza virus and transmissible gastroenteritis coronavirus, therefore, it can be used as the therapeutics or health care food for treating and preventing mammal's diseases caused by the infection of influenza virus and transmissible gastroenteritis coronavirus.