A laser oscillator which establishes r-f discharge by applying an r-f voltage to a plurality of discharge regions via a dielectric member in order to realize laser oscillation. The laser oscillator consists of a DC power source (16) and r-f power sources (18, 19, 20, 21) that convert a DC voltage into an r-f voltage. The DC power source (16) and the r-f power sources (18, 19, 20, 21) are coupled together via a filter (30). The filter (30) prevents r-f components from being fed back from the r-f power sources (18, 19, 20, 21), thus keeping the laser oscillator from unstable operation due to parasitic oscillation. Means is further provided to prevent interference between the r-f power sources and the DC power source.