PROBLEM TO BE SOLVED: To provide a safe antibacterial agent having a high antibacterial activity and effects to various bacteria.SOLUTION: This antibacterial agent is provided by containing an S-nitrosated α-acidic glycoprotein or its variant material as an effective ingredient. The α-acidic glycoprotein is an α-acidic glycoprotein variant A substance, Also, the S-nitrosated α-acidic glycoprotein is provided by eliminating at least one sialic acid at a sugar chain terminal. Further, the S-nitrosated α-acidic glycoprotein variant A substance of which at least one sialic acid at the sugar chain terminal is removed, is also provided.