The present invention provides a method for conveniently producing a protein formulation in which viruses are inactivated, without impairing the quality of the obtained protein formulation, characterized by including the step of exposing the protein formulation contaminated with the viruses to a 0.1-2M aqueous solution of arginine, an arginine derivative, or a mixture thereof, the aqueous solution being adjusted to pH 3.5 to 5. The present invention also provides a virus inactivation method characterized by including the step of contacting a virus-containing object with a 0.1-2M aqueous solution of arginine, an arginine derivative, or a mixture thereof, the aqueous solution being adjusted to pH 3.5 to 5.