A method for determining whether a test sample contains a phytopathogenic fungus, which includes (a) putting the test sample on a front surface of a substrate having a through hole; the substrate having a cellulose film on the back surface thereof; the through hole has a cross-sectional area of not less than 7.065 square micrometers and not more than 19.625 square micrometers; and the cellulose film has a thickness of not less than 0.5 micrometers and not more than 3.7 micrometers; (b) leaving the test sample at rest after the step (a); (c) irradiating the cellulose film with ultraviolet light after the step (b); (d) bringing a back surface of the cellulose film into contact with a fungus color reaction reagent after the step (c); and (e) determining that the test sample contains the phytopathogenic fungus, if a color is given to the fungus color reaction reagent.