To provide a laser amplifier, a laser amplifying method, a laser amplifying device and a laser oscillator wherein pumping by a semiconductor laser is enabled and high efficiency, small size, long life and highly stable operation can be simultaneously realized, in a laser amplifier into which rare earth element for forming self-terminal system transition is added, and to provide a laser amplifying method, a laser amplifying device and a laser oscillator.

This laser amplifier is achieved by using a semiconductor laser light source having two properly selected wavelength as a pumping light source. For the pumping light source, a first semiconductor laser pumping light source for exciting ions from a ground level up to a laser lower level or an energy level higher than the laser lower level, and a second semiconductor laser pumping light source which has wavelength different from the wavelength of the first pumping light source and excites ions from the laser lower level up to a laser upper level, are used.