A method of detecting a fungus belonging to genus Geosmithia, including identifying a fungus belonging to genus Geosmithia using a nucleic acid represented by the nucleotide sequence defined in the following (a) or (b): (a) a partial nucleotide sequence of β-tubulin gene shown in any one of SEQ ID NOS: 1 to 3, or a complementary sequence thereof; (b) a nucleotide sequence including deletion, substitution, insertion or addition of one or several nucleotide(s) in the nucleotide sequence shown in any one of SEQ ID NOS: 1 to 3, or a complementary sequence thereof.