A method for filtrating a microbial culture solution using a membrane module, the method comprising: (a) a step of performing cross-flow filtration of the microbial culture solution using the membrane module; (b) a step of performing water rinsing of the membrane module after the step a; (c) a step of bringing the membrane module into contact with a chemical solution after the step b; and (d) a step of performing water rinsing of the membrane module after the step c, wherein a filtrate that has permeated through the membrane module by the cross-flow filtration in the step a has a total sugar concentration of 1,000 mg/L to 100,000 mg/L, determined according to the description and a protein concentration of 50 mg/L to 1,000 mg/L, determined according to the description, the step c includes a step (c-1) of bringing the membrane module into contact with a chemical solution containing hypochlorite and a nonionic surfactant, and the steps a to d are repeated in this order, and wherein, in the cross-flow filtration in the step a, a membrane surface linear velocity of the microbial culture solution is set to be 0.1 m/s to 3.0 m/s and a filtration flux is set to be 0.1 m 3 /m 2 /d to 2.0 m 3 /m 2 /d, wherein the hypochlorite has an effective chlorine concentration of 0.05% by mass to 1% by mass, and the nonionic surfactant has a nonionic surfactant concentration of 0.05% by mass to 3% by mass and a hydrophilic-lipophilic balance of 13 to 17, wherein the chemical solution has a pH of 10 to 14, and wherein the step c has a step which is operated within a range where a product CT [(mg/L) · h] of a hypochlorite concentration C [mg/L] in the chemical solution and a contact time T [h] between the hypochlorite and a separation membrane contained in the membrane module satisfies Expression: 9.2 × 10 5 F − 1400 ≤ CT ≤ 2.5 1 / α + 380 in Expression, F represents a ratio of the protein concentration [mg/L] in the filtrate which has permeated through the membrane module by the cross-flow filtration in the step a with respect to a total organic carbon amount [mg/L] in the filtrate, and α represents an absolute value [1/h] of an attenuation rate of a membrane strength initial value ratio with respect to an immersion time in a case where the separation membrane is immersed in a Fenton's reagent which is a mixed solution of 5,000 ppm of H 2 O 2 and 300 ppm of Fe 2+, the total organic carbon amount and the attenuation rate α being determined according to the description. (a) a step of performing cross-flow filtration of the microbial culture solution using the membrane module; (b) a step of performing water rinsing of the membrane module after the step a; (c) a step of bringing the membrane module into contact with a chemical solution after the step b; and (d) a step of performing water rinsing of the membrane module after the step c, the step c includes a step (c-1) of bringing the membrane module into contact with a chemical solution containing hypochlorite and a nonionic surfactant, and the steps a to d are repeated in this order, and wherein, in the cross-flow filtration in the step a, a membrane surface linear velocity of the microbial culture solution is set to be 0.1 m/s to 3.0 m/s and a filtration flux is set to be 0.1 m 3 /m 2 /d to 2.0 m 3 /m 2 /d, wherein the hypochlorite has an effective chlorine concentration of 0.05% by mass to 1% by mass, and the nonionic surfactant has a nonionic surfactant concentration of 0.05% by mass to 3% by mass and a hydrophilic-lipophilic balance of 13 to 17, wherein the chemical solution has a pH of 10 to 14, and wherein the step c has a step which is operated within a range where a product CT [(mg/L) · h] of a hypochlorite concentration C [mg/L] in the chemical solution and a contact time T [h] between the hypochlorite and a separation membrane contained in the membrane module satisfies Expression: 9.2 × 10 5 F − 1400 ≤ CT ≤ 2.5 1 / α + 380 in Expression, F represents a ratio of the protein concentration [mg/L] in the filtrate which has permeated through the membrane module by the cross-flow filtration in the step a with respect to a total organic carbon amount [mg/L] in the filtrate, and α represents an absolute value [1/h] of an attenuation rate of a membrane strength initial value ratio with respect to an immersion time in a case where the separation membrane is immersed in a Fenton's reagent which is a mixed solution of 5,000 ppm of H 2 O 2 and 300 ppm of Fe 2+, the total organic carbon amount and the attenuation rate α being determined according to the description.