Provided is a filtration method for the step of removing viruses in a process of producing a protein preparation during which an intermediate protein preparation with a high protein concentration flows at high pressure to a virus removal filter. The filtration method employs a virus removal filter which has a close fitting nozzle of which the inlet at least has a pressure resistance of 600 kPa or more, and an effective area of the virus removal membrane of at least 0.0001 m 2 and no more than 0.03 m 2 . An intermediate protein preparation with the protein concentration raised to at least 20 mg/ml and no more than 100 mg/ml by means of a purification process flows down to said virus removal filter under conditions in which the inlet pressure of the intermediate protein preparation is at least 150kPa and no more than 600kPa, the average protein filtration speed is at least 1.0 kg/m 2 /hr, and the filtration time is at least 1 hour and no more than 5 hours, to give an intermediate protein preparation filtrate with a viral removal rate (LRV) of ‰¥3.
