A temperature detector for use in a high temperature and high pressure furnace, including thermocouple wires received in a tubular protective sheath for protection against the furnace atmosphere, and characterized by the provision of a pair of rod members of a large diameter serving as a thermocouple pair for the positive and negative sides thereof, a protective sheath having a rod suspending holder portion in an upper portion thereof for supporting the thermocouple rod members therein in a vertically suspended state and arranged to hold the rod members out of contact with each other except at a temperature measuring point and to contact the rod members with the protective sheath only in a region other than a high temperature region of the furnace.