PURPOSE: To obtain more laser outputs for the same electric input and to enable to reduce a voltage necessary to start discharging by coating a portion of 2/3 of a discharging length from + pole side of a discharging tube with a tube made of a conductive substance having the same potential as the + pole, and feeding a coolant medium between the tube and the tube made of the conductive material.

CONSTITUTION: A discharging tube 3 is coated with a tube 12 made of a conductive substance having the same potential as a + pole 7, and a coolant medium 13 is fed between the tube 13 and the tube 3. The length of the tube 12 is 2/3 of the discharging tube length. The coolant medium is fed from a gas outlet side 6 through the portion of 2/3 or less of the discharging tube to cool the portion in which gas temperature rise is vigorous to prevent the gas temperature from rising, thereby increasing laser output for the same electric input. The portion of 2/3 or lower of the pole + pole 7 of the tube 3 is coated with the tube 12 having the same potential to ionize the laser medium in the tube 3 by a high voltage, thereby reducing a voltage necessary to start discharging.