Radiation therapy using proton is researching actively these days based on the merit superior than other radiation therapy. In Korea, first proton therapy facility will be opened in 2007 and start to treat solid cancers including no curable cancer until now. In spite of many merits, proton therapy facility has an additional radiation problem. At the aspect of radiation protection, analyzing and understanding of additional neutron dose is major motivation of this study. In order to achieve this, computer simulation with MCNPX(ver. 2.5) and radiation measurement are conducted. Object site is National Cancer Center and concerning area is confined to fixed beam treatment room. Simulated nozzle was tested by NIST proton range data for ensuring simulation result. Neutron measurement is executed with Bubble and He-3 wide energy neutron detectors under same condition to the simulation. Simulation results are showing 0.01 to 1.3 % of additional neutron dose by distance. Contribution of structural materials led to 1.33 to 2.13 times to result of nozzle alone and dominant neutron generating sources were First scatterer, Range modulator, Second scatterer, Snout and Aperture. In the comparison to the other researchers, our results showed a good similarity. Even though additional dose originated from medical purpose is inevitable, study for radiation exposure optimization about this topic should be accomplished continuously in the future.