An object of the present invention is to provide an air filter medium which includes heat-expandable particles that, even when contained in a small amount, impart excellent flame retardancy to the air filter medium and which has excellent flame retardancy and a low pressure loss. The present invention provides an air filter medium including: a plurality of stacked fibrous sheets； and heat-expandable particles and a binder disposed in at least one of interlaminar spaces between adjacent layers of the fibrous sheets.
