A method of precisely measuring temperature of an object without having to setting the emissivity of the object over a wide temperature range from low to high temperature with a single radiation thermometer. The temperature-measuring device of the present invention includes a reference object having an emissivity of substantially 1 in a prescribed wavelength range; a bandpass filter transmitting radiant energy in the prescribed wavelength range; and a radiation thermometer for observing temperature by taking in the radiant energy transmitted through the bandpass filter. The device has a structure for measuring temperature of an object by bringing the reference object into contact with the object, observing temperature radiated from the reference object after the temperature of the reference object has become substantially in the thermal equilibrium with the object and transmitted through the bandpass filter using the radiation thermometer, and determining the object temperature based on the observed temperature.