PURPOSE: To prevent the degradation of collecting efficiency of an electret filter medium by arranging a gas absorbing means at the upstream side of the electret filter medium in an air filter device made by attaching the electret filter medium.

CONSTITUTION: This air filter device is formed by arranging the gas absorbing means 2 at the upstream side of the electret filter element 1. The air filter device is formed by integrally fixing an active carbon fiber nonwoven cloth 2a at the upstream side of the electret filter medium 1 tucked in pleat state in the case of using, for instance, the active carbon fiber nonwoven cloth 2a as the gas absorbing means 2. Thus, the degradation of collecting efficiency with time is extremely prevented by removing gas components in the normal out door air with the gas absorbing means 2 arranged at the upstream side and, after that, by passing through the electret filter medium 1.