A composition for pickling a welded portion and a rust-generated portion according to installation of pipes, structures, plants, and so on formed of stainless steel, and for forming a passive state film thereon is provided. More particularly, the present disclosure relates to a neutral agent for passivating a surface of stainless steel metal in which a neutral agent is obtained by neutralizing a basic aqueous solution to 6.9 pH to 7.1 pH and aerated with carbon dioxide-free air to increase the amount of dissolved oxygen, so that an oxidation reduction potential in a range of 170 mV to 310 mV is obtained.