Contaminant viruses can be efficiently removed almost without losing the activity of protein by subjecting a plasma protein composition having a high risk of viral contamination to a treatment with a porous membrane having a pore size greater than a single-particle size of the virus, particularly by subjecting a plasma protein composition to a fractionation treatment by precipitation, before the porous membrane treatment. Particularly, a fibrinogen composition substantially free of non-enveloped viruses, Parvovirus among others, can be provided. By the application of the present invention, a safe plasma protein preparation free of viruses can be conveniently provided.