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A relativistic equation is deduced for the bound state of two particles, byassuming a proper boundary condition for the propagation of the negative-energystates. It reduces to the (one-body)Dirac equation in the infinite limit of oneof the constituent mass. It also has the symmetries to assure the existence ofthe anti-bound-state with the same mass. The interactionkernel(pseudo-potential) is systematically constructed by diagonalizing theHamiltonian of the background field theory, by which the retardation effectsare included in the interaction. Its wave function at the origin(WFO) behavesproperly in a manner suggested by the covariant field theory.  The equation can well be applied in the heavy quark effective theory. It canalso be used as a starting equation for the unperturbed system in the rigorousperturbation theory of QED.Comment: LaTeX, 12 page

Two-body Dirac equation and its wave function at the origin