A slab type laser apparatus has a slab type gas laser medium part formed in a region defined by a pair of electrode flat plates oppositely disposed in parallel with each other in a space to be filled with a gas laser medium which is excited by high-frequency electric power. The apparatus includes an oscillator part including a pair of resonator mirrors oppositely disposed with a part of the gas laser medium part in between, and for amplifying a laser beam to have predetermined light intensity to emit the laser beam, and the amplifier part including a plurality of return mirrors oppositely disposed with a part of the gas laser medium part in between. The incident laser beam goes and returns plural times between the return mirrors, and the laser beam is amplified to have predetermined power.