A method for manufacturing a virus-reduced protein drug, comprising the following step (a): (a) a filtration step of filtering a virus-containing protein solution through a small-pore size virus removal membrane, wherein the small-pore size virus removal membrane has a pore size that does not permit permeation of viruses but permits permeation of protein molecules, - the filtration step (a) comprising the following step (p) performed prior to the low-pressure fltration step (q): (p) a high-pressure filtration step of filtering the virus-containing protein solution through the small-pore size virus removal membrane at a filtration pressure exceeding 29.4 kPa (0.30 kgf/cm 2), and - the filtration step (a) comprising the following step (q): (q) relieving the filtration pressure after step (p), and a low-pressure filtration step of filtering a washing solution or the virus-containing protein solution through a small-pore size virus removal membrane at a filtration pressure of 29.4 kPa (0.30 kgf/cm 2), or lower, wherein the washing solution or the virus-containing protein solution prior to filtration in the low-pressure filtration step (q) has a pH X and a salt ionic strength Y that satisfy the following equations 1 and 5: 0 ≤ Y mM ≤ 150 X − 590 3.5 ≤ X ≤ 8.0 or the following equations 4 and 5: Y = 0 mM 3.5 ≤ X ≤ 8.0 (a) a filtration step of filtering a virus-containing protein solution through a small-pore size virus removal membrane, wherein the small-pore size virus removal membrane has a pore size that does not permit permeation of viruses but permits permeation of protein molecules, - the filtration step (a) comprising the following step (p) performed prior to the low-pressure fltration step (q): (p) a high-pressure filtration step of filtering the virus-containing protein solution through the small-pore size virus removal membrane at a filtration pressure exceeding 29.4 kPa (0.30 kgf/cm 2), and - the filtration step (a) comprising the following step (q): (q) relieving the filtration pressure after step (p), and a low-pressure filtration step of filtering a washing solution or the virus-containing protein solution through a small-pore size virus removal membrane at a filtration pressure of 29.4 kPa (0.30 kgf/cm 2), or lower, wherein the washing solution or the virus-containing protein solution prior to filtration in the low-pressure filtration step (q) has a pH X and a salt ionic strength Y that satisfy the following equations 1 and 5: 0 ≤ Y mM ≤ 150 X − 590 3.5 ≤ X ≤ 8.0 or the following equations 4 and 5: Y = 0 mM 3.5 ≤ X ≤ 8.0 - the filtration step (a) comprising the following step (p) performed prior to the low-pressure fltration step (q): (p) a high-pressure filtration step of filtering the virus-containing protein solution through the small-pore size virus removal membrane at a filtration pressure exceeding 29.4 kPa (0.30 kgf/cm 2), and (p) a high-pressure filtration step of filtering the virus-containing protein solution through the small-pore size virus removal membrane at a filtration pressure exceeding 29.4 kPa (0.30 kgf/cm 2), and - the filtration step (a) comprising the following step (q): (q) relieving the filtration pressure after step (p), and a low-pressure filtration step of filtering a washing solution or the virus-containing protein solution through a small-pore size virus removal membrane at a filtration pressure of 29.4 kPa (0.30 kgf/cm 2), or lower, wherein the washing solution or the virus-containing protein solution prior to filtration in the low-pressure filtration step (q) has a pH X and a salt ionic strength Y that satisfy the following equations 1 and 5: 0 ≤ Y mM ≤ 150 X − 590 3.5 ≤ X ≤ 8.0 or the following equations 4 and 5: Y = 0 mM 3.5 ≤ X ≤ 8.0 (q) relieving the filtration pressure after step (p), and a low-pressure filtration step of filtering a washing solution or the virus-containing protein solution through a small-pore size virus removal membrane at a filtration pressure of 29.4 kPa (0.30 kgf/cm 2), or lower, wherein the washing solution or the virus-containing protein solution prior to filtration in the low-pressure filtration step (q) has a pH X and a salt ionic strength Y that satisfy the following equations 1 and 5: 0 ≤ Y mM ≤ 150 X − 590 3.5 ≤ X ≤ 8.0 or the following equations 4 and 5: Y = 0 mM 3.5 ≤ X ≤ 8.0