  Spinless Salpeter equation for two bound particles is analyzed. We use the
fact that in relativistic kinematics the spatial two particle relative momentum
is relativistic invariant. Free particle hypothesis for the bound state is
developed: comstituents move as free particles inside of the system. The
Shr\"odinger-type wave equation is derived. Three equivalent forms of the
eigenvalue equation are given. Relative motion of quarks in eigen states is
described by the asymptotic solution in the form of the standing wave of
$\cos(kx+a)$ for each spatial degree of freedom. To test the model the spin
center-of-gravity energy levels for the hydrogen atom are calculated and
compared with the NIST data. Complex eigenmasses for the $H$ atom are obtained.
