PURPOSE: To obtain the titled filter paper which can be burned, having high γ-ray resistance and having high collection efficiency by mixing α-cellulose having 0.1W1μ diameter, a glass fiber having 0.1W0.2μ diameter and a 0.1W1.5-denier synthetic fiber monofilament and making the filter paper from the mixture.

CONSTITUTION: From 10W30% extremely thin fibrillar α-cellulose having 0.1W1μ mean fiber diameter is used. Consequently, the filter paper for an air filter having a comparatively small amt., low pressure drop and high collection efficiency of ≥99.999% is obtained. Besides, glass fiber having 0.1W0.2μ mean fiber diameter is incorporated. A 0.1W1.5-denier synthetic fiber monofilament is further incorporated. Consequently, toughness, γ-ray resistance and UV-ray resistance can be obtained. Polyester and an acryl-PVC copolymer are used as the synthetic fiber to make the filter paper and the filter paper having excellent fire resistance can be obtained.