[en] Experimental studies of neutron and gamma-radiation skyshine at nuclear reactor are proceeding in cooperation with Russia, Kazakhstan and Japan as a project of international science technology center (ISTC). Fast neutron streaming from the vertical experimental hole of IVG.1M reactor which has a cylindrical core are analyzed by a monte carlo n-particle transport code (MCNP) with variance reduction methods, in which a weight window method and a cell importance method can be selected. Calculation results on radial distribution of fast neutron flux at 100 cm above the reactor is compared with the experimental values. The calculated values of neutron flux by using the cell importance method, however, is very different from the experimental values at close distance of 10 cm from the center. Skyshine analysis of neutron radiation streaming from the reactor are also carried out by the equivalent source model in which a point source and the detectors are located at 10 cm and 1 m above the ground, respectively. The calculated values of total neutron flux distribution are very close to the experimental values. The effects of the air composition on neutron flux calculation are also investigated. (M. Suetake)