PURPOSE: To capture and remove ≤0.1μm fine particles from air, by the use of an ultrafiltration membrane having ventilative fine pores and ≤100,000 fractional MW as an air filter.

CONSTITUTION: An ultrafiltration membrane of polysulfone resins, polyimide resins having ventilative fine pores and ≤100,000 fractional MW is used as an air filter. By using the membrane, ≤0.1μm fine particles further about 0.01μm-degree particles are captured and removed from an atmosphere though it was hard conventionally. Even if a conventional ultrafiltration membrane with fractional MW ≤100,000 for liquids is used in a gas phase, the membrane does not transmit any gases, e.g. air, at all. However, after immersing a conventional wet ultrafiltration membrane having fine pores on the surface in ≥80° hot water or pressured hot water or exposing to steam atmosphere and then drying, the pores become practically ventilative pores and the membrane becomes usable for an air filter.