We have investigated the magnetic phase diagrams for anti ferro-quadrupole (AFQ) phase II in CexLa1-xB6 by ultrasonic measurements under high magnetic fields up to 30 T by using the hybrid-magnet (Gama) at National Institute for Materials Science in Tsukuba. Ce0.50La0.50B6 reveals that the AFQ phase boundary in applied magnetic fields along the [0 01] direction closes at about 29 T and the boundaries in fields along the [110] and [111] directions are still open even at 30 T. The AFQ phase transition temperature in Ce0.75La0.25B6 shows maximum at about 25 T in H parallel to [001]. These experimental results support the idea that the multipole RKKY-type inter-site interaction of the quadrupoles O-yz, O-zx, O-xy and the field-induced octupole T-xyz play an important role in the highly anisotropic magnetic phase diagram of phase II of CexLa1-xB6 in high magnetic fields. (c) 2005 Elsevier B.V. All rights reserved.