It is shown that the composite model of elementary particles proposed by Sakata can be formulated as a covariant theory of bound states. The forces among fundamental particles (nucleons and Λ-particles) from which the composite pions, heavy mesons and hyperons are supposed to be constructed are derived from the assumed basic interactions of Fermi type. Equations for bound states are solved by the lowest order approximation (the chain approximation) with the cut-off device.