To stably and efficiently manufacture filter paper for an air filter with a transmissivity of 0.03% or less and a pressure loss of <225 Pa by preventing the accumulation of the eluted matter from glass fibers generated by circulating and reusing used papermaking water, in a method for forming filter paper from a fiber material containing fibers by a wet papermaking process using acidic papermaking water.

In the method for manufacturing filter paper for the air filter, used papermaking water is subjected to ultrafiltration membrane separation treatment to be circulated and used.