  A new technique for constructing the relativistic wave equation for the
two-body system composed of the spin-1/2 and spin-0 particles is proposed. The
method is based on the extension of the SL(2,C) group to the Sp(4,C) one. The
obtained equation includes the interaction potentials, having both the
Lorentz-vector and Lorentz-tensor structure, exactly describes the relativistic
kinematics and possesses the correct one-particle limits. The comparison with
results of other approaches to this problem is discussed.
