To secure the sufficient stability of matter, the Law of Conservation of Heavy Particles is postulated. From this postulate, a sort of intrinsic charge called nucleonic charge is assigned to every elementary particle uniquely (§2). In analogy to electric charge, our law can b e stated as the conservation law of nucleonic charge and the properties of this charge are investigated (§3). Availing of this charge and recently obtained knowledge about heavy mesons, the possible existence of families of elementary particles is presented. Discussions about the interactions between families which seem to have primary importance are made in §4. Bearing the conservation law of nucleonic charge in mind, the production mechanism of heavy mesons is analysed, and the necessity of the ordering of the interactions between families is stressed (§5). In appendix I, some speculations about heavy mesons and in appendix II, the threshold energies for productions are discussed.