We investigate the pressure effect of the electrical resistivity and magnetization of Ce0.8La0.2B6. The situation in which phase IV stably exists inside phase II at H=0 T could be realized by applying a pressure above P∼1.1 GPa. This originates from the fact that the stability of phase II under pressure is larger than those of phases IV and III. The results seem to be difficult to reproduce by taking the four interactions of Γ5u-type AFO, Oxy-type AFQ, Txyz-type AFO, and AF exchange into account within a mean-field calculation framework.