The present invention provides a biological substance detection method for specifically detecting a biological substance from a pathological specimen, by which method, when immunostaining using a fluorescent label and staining for morphological observation using a staining agent for morphological observation are simultaneously performed, the results of fluorescence observation and immunostaining can be assessed properly even if the fluorescent label and/or the staining agent is/are deteriorated by irradiation with an excitation light. The biological substance detection method according to the present invention is characterized in that the brightness retention rate of an immunostained part is in a range of 80% to 120% in relation to the brightness retention rate of a part stained for morphological observation when the fluorescent label used for the immunostaining is observed.