To provide a washing device for an air filter unit capable of improving operability and treatment efficiency with a simple constitution capable of attaining miniaturization and space-saving.

The space between an ultrasonic washing tank 13 and a treatment tank 15 in the following stage is provided with a joint board 14 for temporarily placing an air filter unit 2c. Either conveying apparatus 20 performs the conveyance of air filter units 2a, 2b, 2c, 2s and 2x only in the first-half stage till the air filter units 2a, 2b, 2c, 2s and 2x are placed on the joint board 14. The other conveying apparatus 30 performs the conveyance of the air filter units 2c, 2d, 2e, 2t and 2y only in the latter-half stage after the placing of the air filter units 2c, 2d, 2e, 2t and 2y on the joint board 14.