We present an effective Hamiltonian for cerium monopnictides CeX (X=P,As,Sb,Bi) to explain their complex magnetic phase diagrams in a unified way. The effective Hamiltonian consists of semimetallic conduction bands and localized f spins. Between the conduction electrons and the localized f spins two kinds of interactions work. One is the intraband mechanism which is the usual Kondo exchange interaction and the other is the interband spin-dependent exchange interaction specific to this model. Owing to the competition between the above two interactions, various types of magnetic structures are realized. The ground-state magnetic phase diagram of this model is investigated by a simple mean-field treatment in one dimension and it is shown that the essential features of the complex ground-state magnetic phase diagram of CeX are reproduced.