Pure copper and Fe-15Cr-2ONi alloy were irradiated at 500°C by 5 MeV Ni ions (single beam), Ni ions plus gas atoms (H and He) (dual beam irradiation) or triple beam of Ni, He and H ions (triple beam irradiation) simultaneously. Irradiation was carried out with accelerators TIARA at Takasaki-establishment of JAERI. Thin specimens were prepared with the combined technique of FIB (Focus Ion Beam) and electro-polishing. Damage structure versus the depth from surface was examined by TEM. In Fe-15Cr-20Ni alloy, voids were formed in the range of Ni ion irradiated specimens. In Fe-Cr-Ni alloy which is irradiated by a dual beam of Ni ion and He ion, the number density of voids increased significantly at the stopping range of He atoms. Remarkable results are void formation in specimens irradiated with dual beam of Ni and H ions, only voids of very small size were observed through the range of specimen. In irradiated specimen with triple beam, the size of voids was suppressed to be smaller than that of irradiated specimens with Ni and He ions. In pure copper which is irradiated with dual beam of helium and Ni ions, number density of voids were increased significantly. While in copper irradiated with hydrogen and Ni ions, number density of voids were not so large. Experimental results show that helium atoms have strong effect for void formation, while hydrogen atoms suppress void formation because the remarkable degradation of movement of small vacancy clusters can be induced by trapping hydrogen atoms.