A 14-MeV neutron streaming through a narrow vertical hole duct in the diagnostics room of a Deuterium-Tritium Fusion Experimental Device (R tokamak) was analyzed using the Monte Carlo coupling technique. Neutron dose rate distributions in the horizontal direction as well as in the axial direction along the vertical hole duct were calculated to evaluate the neutron streaming effect through the hole duct. The dose rate distribution in the axial direction undergoes relatively small changes, but the distribution changed abruptly in the horizontal direction. Compared to ANISN results, Monte Carlo calculations show a neutron streaming effect at locations beyond the vertical hole duct axis in the horizontal direction. The fractional standard deviation (FSD) due to error propagation was calculated by the ORION code based on an error propagation equation. The FSDs were within 0.06 at the detector locations along the axial direction along the vertical hole duct; but, they were as much as 0.25 to 0.47 for >15 cm beyond the hole duct axis in the horizontal direction.