PURPOSE: To facilitate the temperature measurement of a discharge tube without giving influence upon the discharge during the laser oscillation and achieve the operation under the optimum conditions by a method wherein an ultrasonic thermometer which measures the temperature of the discharge tube with the propagation speed of the ultrasonic wave is provided.

CONSTITUTION: An ultrasonic thermometer 22 provided outside an outer shell 16 is composed of a pulse generator 25 generating an electric pulse signal, a magnetostrictive element 24 generating an ultrasonic pulse in accordance with the pulse signal from the pulse generator 25, an ultrasonic pulse propagation line 23 through which the generated ultrasonic pulse is inputted to and outputted from a discharge tube 11 and a signal processor 26 receiving the reflected wave of the ultrasonic pulse from the discharge tube 11 which is converted into an electric signal by the magnetostrictive element 24. An echo signal created by the reflected wave of the ultrasonic pulse is inputted by the signal processor 26 and the time interval between the echo signal pulses is measured to measure the temperature of the discharge tube 11.