To provide an air filter of which the filter medium is easily replaced, without the danger of losing the component such as a lid side plate or mistaking the attaching direction of the lid side plate.

To the upper end part of the left and right side plates 2 of the lid side plate 2a, a connection pin 6 shifted to the rear surface side of a casing 3 and a locking pin 7 shifted to the front surface side are attached. To both left and right end parts of the lid side plate 2a, a connection member 9 provided with a J-shaped oblong hole 8 to which the connection pin 6 is to be fitted and a forked pawl member 10 to which the locking pin 7 is to be fitted are attached respectively matched with the front-to-back direction positions of the connection pin 6 and the locking pin 7. By detaching the locking pin 7 from the pawl member 10, turning the lid side plate 2a and guiding the connection pin 6 along the oblong hole 8 to a tip side, the turned lid side plate 2a is withdrawn from the entire surface of a filter medium take-in/take-out port and the filter medium is easily replaced. Also, the loss of the component such as the lid side plate 2a or and the error of the attaching direction of the lid side plate 2a are prevented.