To simplify work for retrofitting a filter unit into a space in a ceiling, enable uniform work, reduce waste, eliminate work for restoring the ceiling, and reduce costs.

This filter unit is equipped with: a box-shaped unit body part; at least one inflow piping connecting port and at least one outflow piping connecting port, which are provided in the unit body part; a jig for fixing piping around the piping connecting ports inside the unit body part; a filter which is arranged inside of the unit body part so as to inhibit the inflow and outflow sides of the piping connecting ports from communicating directly with one another; and a door for inspection, which is openably/closably provided on the downside of the unit body part in such a manner as to be flush with a ceiling surface. The filter unit can be inserted midway through an air- conditioning duct, and the door for the inspection can be opened/closed from the ceiling surface.