Quantized conductance is reported in high-crystalline tin oxide (SnO2) nanobelt back-gate field-effect transistors, at low temperatures. The quantized conductance was observed as current oscillations in the drain current vs. gate voltage characteristics, and were analyzed considering the nanobelt as a quantum wire with rectangular cross-section hard-walls. The quantum confinement in the nanowires created conditions for the successive filling of the electron energy-subbands, as the gate voltage increases. When the source-drain voltage is changed the oscillations are not dislocated with respect to Vg, indicating flat-band subband energies at low temperatures. The subband separation was found to be in good agreement with the experimental observations, since the oscillations tend to disappear for T > 60K. Therefore, a novel quantum effect is reported in SnO2 nanobelts, which is expected to behave as bulk at zero electric gate fields.