A method of easily detecting a slight percentage of seeds contaminated with a seed-transmissible pathogen as mixed in among a large number of seeds is provided. The method comprises placing a substratum for germination with seeds disposed thereon in a hermetic container, adding an extractant to the hermetic container after germination of the seeds, mixing up the seedlings resulting from germination of the seeds, the germination substratum and the extractant with stirring and/or pressing, and using the mixed liquid derived therefrom as a test material for detecting pathogenic bacteria possibly occurring therein.