An air filter 100 includes a plurality of filter units 2 and an outer frame 10 surrounding the plurality of filter units 2. The filter units 2 each include a pleated filter medium 4 and a supporting frame 6 holding a peripheral portion 4e of the filter medium 4. Adjacent two filter units 2 and 2 form a V-shape. The plurality of filter units 2 are coupled to each other at the respective supporting frames 6 thereof. The plurality of filter units 2 are fitted in the outer frame 10 so that all of the filter units 2 are inclined with respect to in-plane directions of opening surfaces of the outer frame 10.