We investigated the formation of nanopores in silicon nitride membrane by means of wet etching. The membrane was irradiated with focused helium ion beam using helium ion microscope, and then etched in hydrofluoric acid. Nanopores were investigated with transmission electron microscope. The pores with a diameter from 10 to 100 nm were obtained. Dependence of the pore diameter on the number of helium ions was investigated and compared with space distribution of radiation-induced defects, which was obtained from Monte-Carlo simulation.