To provide a filter unit which has a high versatility, and can be fixed immediately to air inlets of various shapes and sizes in the existing buildings, and an air inlet of a building provided with such a filter unit.

A shaft member 22 is inserted into substantially the center of a cylindrical filter material 21, and is expandably formed in a radial direction, and the filter material receives the power from the expanded shaft member to be formed so as to expand in the radial direction. In the filter unit 20 thus formed, the shaft member expands after inserted into the inside of an opening 11 of the air inlet, by which the cylindrical filter material is pushed, and expands in the radial direction to attach firmly to the opening surface. The air inlet of the building is thus formed.

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