To provide a washing device for an air filter unit capable of easily miniaturization/space-saving with a simple constitution and enhancing workability and treatment efficiency.

The washing device for an air filter unit is provided with a plurality of treatment tanks 32, 34, 36, 38 including an ultrasonic washing tank 34 arranged between a first inversion device 26 and a second inversion device 42 in a single row; and a conveying device arranged along the plurality of treatment tanks 32, 34, 36, 38. The first inversion device 26 receives the air filter unit 12a in the state that a major surface becomes approximately horizontal and stands at approximately right angles and makes the major surface approximately vertical. The conveying device grasps the air filter unit 12b in which the major surface becomes approximately vertical and conveys the air filter unit 12b to the respective treatment tanks 32, 34, 36, 38 while retaining the state that the major surface becomes approximately vertical. The second inversion device 42 receives the air filter unit 12b and falls down it at the approximately right angles and the major surface is made approximately horizontal.