To regenerate a used air filter element simply and efficiently in a short time.

The used air filter element is regenerated through a process (a) for washing the used air filter element, a process (b) for dehydrating the washed air filter element, and a process (c) for drying the dehydrated air filter element by dielectric heating. The filtering part of the air filter element is formed by drying a sheetlike filter medium having a large number of pleats formed thereto by bending by microwave dielectric heating.