<p>A filter medium which has an average pore diameter of more than 0.5 mu m, a pressure loss of 2-50 mmH2O determined when air is permeated therethrough at a velocity of flow of 5.3 cm/sec, and a PF value of 18-12, which is calculated in accordance with the following equation on the basis of this pressure loss and a collection efficiency using DOP of a grain size of 0.10-0.12 mu m. PF value = [ -log (permeability (%)/100) ] / [ pressure loss ] * 100. Permeability (%) = 100 - collection efficiency (%).</p>
