Since micellization typically occurs at small concentrations, existing theories imply an ideal behavior of micellar systems near the critical micelle concentration (CMC). However, micellization also occurs at not small but rather moderate concentrations, which needs a certain generalization in theory. In this paper, the theory based on the definition of the CMC via the constant of the mass action law is generalized for relatively concentrated systems where a set of micelles and monomers exhibits a nonideal behavior. The theory is restricted by a concentration range above the CMC where aggregation numbers can be considered as constants. Being more complicated, the case of a nonideal micellar system possesses its own simplifications. First, expressing all concentrations in the CMC units, the activity coefficient f is replaced by f/f_m where f_m is the f-value at the CMC. This ratio can be almost constant or unity even for a nonideal system. The second simplification is related to forming electroneutral micelles in a concentrated ionic solution. The theory of such systems is developed in detail including explicit expressions for micellar and monomeric concentrations in terms of aggregation numbers. One more simplification was discovered when applying the theory to the diffusion of surfactants in micellar solutions. This is a quasi-ideal behavior of a relatively concentrated ionic micellar system caused by the mutual compensation of the viscosity and nonideality effects in the course of diffusion.