Disclosed is a probe for detecting a substance in the body in gingival crevicular fluid. The probe includes a gingival sulcus insert section and a sensitive part. The gingival sulcus insert section can be inserted into a gingival sulcus. The sensitive part is arranged on the gingival sulcus insert section and contains a detection substance for permitting optical detection of the substance in the body. A system for detecting a substance in the body is also disclosed. The system for detecting a substance in the body includes the probe for detecting a substance in the body, a light irradiator for irradiating light onto the sensitive part in the probe, and a photodetector for detecting optical information from an inside of the sensitive part upon irradiation of light by the light irradiator.