PURPOSE: To enhance efficiency by a method wherein movable ends of piezo elements are installed at three or four points in the periphery of a reflection mirror for a light wave resonator, and a voltage is so impressed on each piezo element that a laser output becomes the maximum.

CONSTITUTION: Discharge tubes 2 and 3, control circuit boxes 8 and 15, and a reaction furnace 21 are installed on a substrate 1, the reflection mirror 13 is installed at the movable end of the piezo elements 10∼12 whose one end is fixed on the control circuit box 8, and a reflection mirror 18 is installed on the movable end of the piezo element 17 whose one end is fixed on the control box 15. The voltage is impressed on each piezo element 10∼12 from the control box 8, resulting in the drive of the piezo elements. The combination of the voltage impressed on each piezo element is differently varied, thus detecting the oscillation output at that time by means of a photoelectric element 14 through the reflection mirror 13 and then judging the combination of the control voltage whereby the oscillation output becomes the maximum, and accordingly the control voltage is continued to be fed to each piezo element.