To provide a technique allowing an in-vivo substance in gingival crevicular fluid to be measured in real time, with high accuracy, and in noninvasive manner.

The present invention provides an in-vivo substance detection probe, which is an instrument for detecting the in-vivo substance present in the gingival crevicular fluid and has a gingival crevice inserting part insertable into a gingival crevice. The gingival crevice inserting part is provided with a sensing section containing a substance for detection which enables the in-vivo substance to be detected optically. According to the invention, the in-vivo substance in the gingival crevicular fluid can be measured in the gingival crevice, and therefore the in-vivo substance in the gingival crevicular fluid can be measured in real time, with high accuracy, and in noninvasive manner.