  We consider a scalar Yukawa-like model in the framework of partially reduced
quantum field theory. The reduced Lagrangian of the model consists of free
scalar field terms and nonlocal current interaction term. Hamiltonian
expressions for conserved quantities arose from a Lorentz-invariance of the
model in the momentum representation have been found in the first-order
approximation with respect to a coupling constant squared. Canonical
quantization of the system is performed. It is shown that the obtained
conserved quantities and previously founded the Hamiltonian and the momentum of
the system satisfy the commutational relations of the Poincare group. The
expression for S-matrix in the current approximation is found. Unitarity of
this operator is proven by direct calculation.
