The purpose of the present invention is to provide a hypochlorous acid aqueous solution having excellent antimicrobial effect and safety. An antimicrobial agent is provided, which is a hypochlorous acid aqueous solution with a pH 6.0 to 6.7, wherein the effective chlorine concentration in the aqueous solution is from 50 to 260 ppm. Further, a sterilization method is provided, which is characterized by immersing the member to undergo sterilization treatment in the aforementioned antimicrobial agent for 0.5 to 10 minutes. The antimicrobial agent has a broad antimicrobial spectrum of norovirus, Staphylococcus aureus and other bacteria, yeast belonging to Rhodotorula, and Cladosporium cladospolioides and other fungi.