  A relativistic equation is deduced for the bound state of two particles, by
assuming a proper boundary condition for the propagation of the negative-energy
states. It reduces to the (one-body)Dirac equation in the infinite limit of one
of the constituent mass. It also has the symmetries to assure the existence of
the anti-bound-state with the same mass. The interaction
kernel(pseudo-potential) is systematically constructed by diagonalizing the
Hamiltonian of the background field theory, by which the retardation effects
are included in the interaction. Its wave function at the origin(WFO) behaves
properly in a manner suggested by the covariant field theory. The equation can
well be applied in the heavy quark effective theory.
