A method for controlling phases of beams to make relative phases of the beams “0 ” in a beam splitting amplification laser using a stimulated Brillouin scattering phase conjugate mirror. A procedure of making a phase difference between laser beams reflected from the phase conjugate mirror “0” comprises a first step of inputting laser beams reflected from the phase conjugate mirrors to a light detector through light path converter a second step of using one of the reflected laser beams as a reference laser beam and making the reference laser beam interfere with another reflected laser beam; and a third step of detecting the interference result and finely driving piezoelectric element to control the positions of the reflectors, to thereby make a phase difference between the become “0”.