Disclosed herein is an air filter device, installed on a line of a pneumatic circuit to remove harmful impurities. The device (10) includes a main body (20) having a casing shape, a cylindrical filter cartridge (30), and a cap (40). The main body includes air inlet and outlet ports (22, 24) on opposite sides thereof. A guide ring (26) is provided adjacent to an inner wall (20a) to have a predetermined height, with a hole (28) formed in the guide ring to directly communicate with the air outlet port. The filter cartridge is accommodated in an inner space (S3) of the guide ring (26), with a filter material (34) horizontally disposed in the filter cartridge. The cap is fastened to an opening of the main body, and includes a spacer (44) which compresses the upper end of the filter cartridge.