A filter medium (1, 2, 3) for an air filter to collect dust in a gas, comprising: a first porous film mainly containing a fluororesin; a second porous film mainly containing a fluororesin, and arranged to the downstream side of the first porous film in an air current; and a pre-collection member to collect a portion of the dust in the air current, the pre-collection member being arranged to an upstream side of the first porous film in the air current, and having a thickness of 0.4 mm or less; characterized in that: the pre-collection member has a pressure drop when air is passed therethrough at a flow rate of 5.3 cm/s of 15 Pa or greater and less than 55 Pa, a collection efficiency of NaCl particles having a particle diameter of 0.3 μm of 25% or greater and less than 80%, and a PF value determined from the following formula: PF value = {-log ((100 - collection efficiency (%))/100)}/(pressure drop (Pa)/1000) of 7 or greater and 15 or less; the filter medium (1, 2, 3) for an air filter has a pressure drop less than 200 Pa when air passes through at a flow rate of 5.3 cm/s, and a collection efficiency of NaCl particle having a particle diameter of 0.3 μm is 99.97% or greater when air containing the particles passes through at a flow rate of 5.3 cm/s; the first porous film and the second porous film mainly comprise fibril-forming polytetrafluoroethylene, a non-fibril-forming non-melting-processable component, and a non-fibril-forming hot-melt-processable component having a melting point below 320°C; the average pore diameter of each of the first porous film and the second porous film exceeds 1.6 μm; and the filter medium (1, 2, 3) for an air filter is configured such that a value of "the PF value of the pre-collection member/a PF value when the first porous film and the second porous film are overlapped," which is a ratio of the PF value of the pre-collection member to the PF value when the first porous film and the second porous film are overlapped, is 0.20 or greater and 0.45 or less. a first porous film mainly containing a fluororesin; a second porous film mainly containing a fluororesin, and arranged to the downstream side of the first porous film in an air current; and a pre-collection member to collect a portion of the dust in the air current, the pre-collection member being arranged to an upstream side of the first porous film in the air current, and having a thickness of 0.4 mm or less; characterized in that: the pre-collection member has a pressure drop when air is passed therethrough at a flow rate of 5.3 cm/s of 15 Pa or greater and less than 55 Pa, a collection efficiency of NaCl particles having a particle diameter of 0.3 μm of 25% or greater and less than 80%, and a PF value determined from the following formula: PF value = {-log ((100 - collection efficiency (%))/100)}/(pressure drop (Pa)/1000) of 7 or greater and 15 or less; the filter medium (1, 2, 3) for an air filter has a pressure drop less than 200 Pa when air passes through at a flow rate of 5.3 cm/s, and a collection efficiency of NaCl particle having a particle diameter of 0.3 μm is 99.97% or greater when air containing the particles passes through at a flow rate of 5.3 cm/s; the first porous film and the second porous film mainly comprise fibril-forming polytetrafluoroethylene, a non-fibril-forming non-melting-processable component, and a non-fibril-forming hot-melt-processable component having a melting point below 320°C; the average pore diameter of each of the first porous film and the second porous film exceeds 1.6 μm; and the filter medium (1, 2, 3) for an air filter is configured such that a value of "the PF value of the pre-collection member/a PF value when the first porous film and the second porous film are overlapped," which is a ratio of the PF value of the pre-collection member to the PF value when the first porous film and the second porous film are overlapped, is 0.20 or greater and 0.45 or less. the pre-collection member has a pressure drop when air is passed therethrough at a flow rate of 5.3 cm/s of 15 Pa or greater and less than 55 Pa, a collection efficiency of NaCl particles having a particle diameter of 0.3 μm of 25% or greater and less than 80%, and a PF value determined from the following formula: PF value = {-log ((100 - collection efficiency (%))/100)}/(pressure drop (Pa)/1000) of 7 or greater and 15 or less; the filter medium (1, 2, 3) for an air filter has a pressure drop less than 200 Pa when air passes through at a flow rate of 5.3 cm/s, and a collection efficiency of NaCl particle having a particle diameter of 0.3 μm is 99.97% or greater when air containing the particles passes through at a flow rate of 5.3 cm/s; the first porous film and the second porous film mainly comprise fibril-forming polytetrafluoroethylene, a non-fibril-forming non-melting-processable component, and a non-fibril-forming hot-melt-processable component having a melting point below 320°C; the average pore diameter of each of the first porous film and the second porous film exceeds 1.6 μm; and the filter medium (1, 2, 3) for an air filter is configured such that a value of "the PF value of the pre-collection member/a PF value when the first porous film and the second porous film are overlapped," which is a ratio of the PF value of the pre-collection member to the PF value when the first porous film and the second porous film are overlapped, is 0.20 or greater and 0.45 or less.