A method and a reagent for detecting cells of a living organism contained in various samples of the organism by detecting genes specific for each animal species. This method is useful for detecting bleeding into feces. It is possible for confirming the bleeding of the human body to use an Alu sequence as the sequence specific for the animal species. As genes are used as the indicator in this method, fecal occult blood can be detected more specifically at a higher sensitivity than by the conventional technique using hemoglobin as the indicator.