We investigate a model for a relativistic particle in the Liouville field.
The model is SL(2, R)/Z(2) invariant. The corresponding dynamic integrals d
escribe the whole set of classical trajectories. These integrals are used f
or the gauge-invariant Hamiltonian reduction. We propose a new scheme for q
uantizing the reduced system. The quantum system obtained reproduces the cl
assical symmetry. We discuss the physical aspects of the model.