PROBLEM TO BE SOLVED: To provide an infection accelerator of mycorrhizal fungus, containing as the active ingredient a chemical component capable of being industrially synthesized or a specific component capable of being efficiently isolated by extraction from the natural world, to provide an infection/growth accelerator of mycorrhizal fungus, and to provide a method for accelerating mycorrhiza formation of plants using such a medicinal agent. SOLUTION: This infection accelerator of mycorrhizal fungus toward plant root contains as the active ingredient 10-1,500 ppm of mannitol. The other objective infection/growth accelerator of mycorrhizal fungus toward plant root infected with mycorrphizal fungus contains as the active ingredient 50-500 ppm of mannitol; wherein the bioactivation of the mycorrhizal fungus is activated, the infectivity of the mycorrhizal fungus toward plant root is increased and also the growth of the hypha is accelerated and plant growth becomes better and also resisting power against disease becomes high.
