To spontaneously discharge the collected oil mist from inside a filter without squeezing, etc., by providing a non-dense coarse-mesh filter respectively on the air inlet side and outlet side and inserting a dense fine-mesh filter between both filters.

Three groups 10 of filters are laminated and housed in a filter chamber 3X connected to a fan casing 1 and provided on one side in a dust collecting chamber 3 and almost vertically to the air current. The two front and rear coarse-mesh non-dense filters 7 and 9 on the air inlet 3T side and outlet side 3B respectively have 500-300 μm openings and are made of urethane foam. The dense intermediate fine-mesh filter 8 closely inserted between the filters 7 and 9 has 20-10 μm opening and is made of kapok fiber.