An air filter medium reduces a decrease in collection efficiency in the case where a main collection layer and a pre-collection layer having a lower collection efficiency than the main collection layer are used. An air filter medium that collects dust in gas includes a main collection layer and a pre-collection layer that has a lower collection efficiency of NaCl particles having a particle size of 0.3 μm than the main collection layer, is disposed on an upstream side of an air flow with respect to the main collection layer, and has water repellency.