To provide a thermometer which can inexpensively measure the temperature of an underground environment with accuracy over a wide extent and, at the same time, with which temperature control can be made easily, and to provide a freezing method using the thermometer.

In the freezing method, a plurality of holes for freezing pipes is formed in parallel along a portion which becomes a tunnel H and freezing pipes are inserted into the holes, respectively. Then the ground G around the freezing pipes is frozen by circulating a cooling liquid in each freezing pipe and the tunnel H is formed by excavating the obtained frozen soil and, at the same time, measuring and controlling the temperature of the ground G. In the method, a thermometer provided with optical fibers 10 which are respectively laid in a plurality of temperature measuring pipes 8 radially arrange toward the ground G from the internal wall F of the tunnel H and a detector 7 which is connected to the optical fibers 10 and detects the wavelength variation of optical signals by causing the optical signals to pass through the fibers 10 is used. By means of the detector 7, the temperature of the underground environment is controlled by measuring temperature changes by detecting the wavelengths of optical signals returned from the optical fibers 10.