We have formulated the mixing Gibbs energy density for the micelle phase of an amphiphilic block copolymer solution using mean-field lattice theory. We have found a transition between the micellization and the liquid-liquid phase separation in this solution by comparing the mixing Gibbs energy density for the micelle phase with that for the homogeneous phase. Micellization is preferred over the liquid-liquid phase separation not only at a higher amphiphilicity of the block copolymer chain but also at a higher degree of polymerization of the copolymer.