boron isotopes in gaseous boron tribromide was 10 b 5 7 and 11 b 94 3 and so 11 b was concentrated in the gas similarly nitrogen monoxide pressure 20 torr volume 120 ml was added to gaseous boron tribromide in natural composition pressure 2 torr volume 120 ml and 4000 pulses of ammonia laser wave length 12 08 u03bc power output 200 mj per pulse were irradiated thereto after irradiation the composition of boron isotopes is gaseous boron tribromide was 10 b 15 0 and 11 b 85 0 and so 11 b was concentrated oxygen pressure 8 torr volume 120 ml was added to gaseous boron tribromide in natural composition pressure 2 torr volume 120 ml and 4000 pulses of ammonia laser power output per pulse wavelength 12 26 u03bc component 150 mj and wave length 12 80 u03bc component 100 mj were irradiated thereto after irradiation the composition of boron isotopes in gaseous boron tribromide was 10 b 32 2 and 11 b 67 8 and so 11 b was concentrated oxygen pressure 8 torr volume 120 ml was added to gaseous boron tribromide in natural composition pressure 2 torr volume 120 ml and ammonia laser power output per pulse wave length 12 26 u03bc component 150 mj and wave length 12 80 u03bc component 100 mj and carbon dioxide gas laser power output per pulse wave length 9 29 u03bc component 200 mj were simultaneously irradiated thereto after irradiation of 3000 pulses the composition of boron isotope in gaseous boron tribromide was 10 b.