A heat monitoring instrument of the invention includes: an insertion portion to be inserted into tissue of a subject； a linear member including a proximal end portion and a distal end portion, in a state in which the insertion portion is inserted into the tissue, the proximal end portion having a predetermined length that extends to outside of the tissue, and the distal end portion being coupled to the insertion portion； and a locking portion that is provided in the insertion portion and engages with the tissue when in a state of being a predetermined temperature or less, and releases engagement with the tissue by changing shape when heated from the state of being the predetermined temperature or less to a predetermined temperature range that exceeds the predetermined temperature.
