PURPOSE: To miniaturize a power supply by lowering voltage required for start ing the discharge of a gas laser device.

CONSTITUTION: On the starting of discharge, a cathode 11 is moved toward an anode 10 by turning and driving motors 17, 17, and an interval d0 between both electrodes 10, 11 is made narrower than that d1 at a time when normal discharge, glow discharge, is maintained. When d0 equals to d1/2, discharge can be started by approximately half of voltage at a time when interval between the electrodes is kept at d1 and discharge is started because breakdown voltage is approximately proportional to the interval between the electrodes. Discharge is started, the motors 17, 17 are contrarotated, and the cathode 11 is retreated under constant-current control until the interval between both electrodes 10, 11 reaches d1.