A method for filtering a protein-containing liquidcontaining protein at a concentration of 20 mg/mL or moreand 100 mg/mL or less, the method including aprefiltration step of filtering the protein-containingliquid by a prefilter having a pore size of 0.08 µm to0.25 µm and including a hydrophobic resin, and a virusremoval step of filtering the protein-containing liquidby a virus removal membrane including a synthetic polymer,after the prefiltration step, wherein the protein-containingliquid before conducting the prefiltrationstep includes 0.25 g or more of a trimer or highermultimer of the proteins having an average diameter ofless than 100 nm, per 1 m2 of the virus removal membrane.
