PURPOSE: To facilitate a measuring preparation and to enable reducing of a construction cost, by a method wherein, from an output of a temperature measuring resistor, being divided into a given resistance value and connected in (n) series, an average temperature is measured through a single temperature converter.

CONSTITUTION: Temperature measuring resistance of (n) pieces 10-1...10-n, located in spots to be measured, are respectively divided into a resistance value of R/n[Ω], where R[Ω]is a given resistance value, and are connected in series. The outputs of the temperature measuring resistors 10-1...10-n, connected in (n) series, are inputted to only a single temperature converter 20, and the output of the temperature converter 20 is inputted to an average thermometer 50. Only one set will suffice for the temperature converter 20, and besides both an adder and a divider can be eliminated. This permits reducing of an installation cost, simplifying of a measuring preparation, and decreasing of a construction cost when compared with conventional measuring method.