A novel nano thermometer, which can be used for temperature measurement of a wide temperature range, in a micrometer size environment, and a method for producing the same. The nano thermometer is a carbon nanotube filled with a continuous columnar gallium, which enables measurement of environmental temperature by length change of the columnar gallium, and is produced by mixing Ga.sub.2O.sub.3 powder and carbon powder uniformly, and performing heat treatment for this mixed powder at a temperature range of 1200 to 1400.degree. C. under an inert gas flow.