The present invention provides a method for determining whether or not a test sample contains a phytopathogenic fungus selectively from two kinds of fungi of a phytopathogenic fungus and a non-phytopathogenic fungus. The method according to the present invention comprises: (a) putting the test sample on a front surface of a substrate comprising a through hole; wherein the substrate comprises a cellulose film on the back surface thereof; the cellulose film has a thickness of not less than 0.5 micrometers and not more than 2 micrometers; and the through hole has a cross-sectional area of not less than 7.065 square micrometers and not more than 19.625 square micrometers; (b) leaving the test sample at rest; (c) observing a back surface of the film; and (d) determining that the test sample contains the phytopathogenic fungus, if a fungus is found on the back surface of the film.