An antitumor agent is provided by including, as an active ingredient, a polysaccharide-glycan complex obtained by treating a Gram-positive bacteria such as lactic acid bacteria and Bifidobacterium strains with a cell wall lytic enzyme produced by a bacterium belonging to the genus Achromobacter.The polysaccharide-glycan complex has no toxicity and causes no side effect. Being water-soluble, it is easily formulated into any arbitrary dose form, such as an injectable solution. It can easily be prepared from an easily available starting material.