Causative microorganisms of infectious diseases are detected and/or identified rapidly and with high sensitivity by taking phagocytes from clinical specimens containing active phagocytes, immobilizing the phagocytes so taken, treating the phagocytes to improve cell membrane permeabilities thereof, further treating the phagocytes to bare DNA in the causative microorganisms which might exist in the phagocytes, and detecting the causative microorganisms with DNA probes which can hybridize with such DNA under stringent conditions.