  It is shown that at least one particle is bound in the $N$-particle
semi-relativistic Pauli-Fierz model with negative potential $V(\bx)$. It is
assumed that the particles have no spin and obey the Bose or Boltzmann
statistics, and the one particle Hamiltonian $\sqrt{-\Delta+M^2}-M+ V(\bx)$ has
a ground state with negative energy $-e_0<0$, where $M>0$ denotes the mass of
the particle. We show that the ground state energy of the total system $E^V(N)$
is less than $E^0(N)-e_0$.
