The beam quality of a single-rod Nd∶YAG laser can be improved almost up to the diffraction limit by using a phase-conjugating mirror (PCM) based on stimulated Brillouin scattering (SBS). As a single Nd∶YAG rod, whose thermal lens, stress birefringence and depolarization are surveyed experimentally, the stable output energy of 20 mJ with pulse duration of 11.8 ns, which has perfect beam quality, is obtained at repetition rate 5 Hz with pump energy of 10 J when the start cavity is the concave-convex cavity operating near a limit of stability in the g diagram and the SBS phase conjugation cavity is optimized.