PURPOSE: To supply a new air filter without stopping the operation of machinery at the tide of the replacement of an air filter in an air filter device having an air filter unit wherein the air filter is received in a case in a folded state.

CONSTITUTION: This air filter device consists of an air filter unit l formed by connecting many air filter pieces 1a in a zigzag shape and folding the formed air filter along the joints in a multilayered state and a before-use receiving part 2a and an after-use receiving part 2b received in a machinery housing 2 in a mutually separated and opposed state at predetermined positions. The air filter pieces 1a of the new air filter unit received in the before-use receiving part is drawn out in a zigzag shape to be used while the clogged air filter pieces are continuously transferred to the after-use receiving chamber to be received therein.