Disclosed is a novel antimicrobial means which is effective against various types of fungi including pathogenic true fungi and phytopathogenic filamentous fungi. An antimicrobial agent against fungi according to the present invention comprises, as an effective ingredient, cyanoacrylate polymer particles having an average particle diameter of less than 1000 nm, which particles contain at least one selected from the group consisting of amino acids, amino acid derivatives, oligomers and polymers thereof, saccharides and polysorbates, and contain no antimicrobial active ingredient effective against the fungi. The antimicrobial agent is particularly useful as a control agent against plant diseases caused by filamentous fungi.