To perform a filtration operation at high pressures stably for a long time. The filtration method uses a hollow fiber membrane module comprising a plurality of bundled hollow fiber membranes inserted into a module case and in which opposing ends of the hollow fiber membranes are integrated by a potting material. In the filtration method, the relationship 0.5<；R/L<；4 is satisfied, where R is % diameter expansion of a center portion in a longitudinal direction of the hollow fiber membrane module and L is % longitudinal elongation of the hollow fiber membrane module, when the internal pressure of the hollow fiber membrane module is set to 1.0 MPa under unconstrained conditions. During operation, % diameter expansion of the center portion is 0.3% or less in a state where the hollow fiber membrane module is longitudinally constrained.
