An air filter device for a video projector, which includes a light source lamp having an optical axis and which is used in a state in which the optical axis of the light source lamp is horizontal. The air filter device includes an air filter and a cleaner that cleans the air filter. The cleaner includes a rotation brush rotated about a rotation axis to remove dust from the air filter while moving along an upstream side of the air filter. The rotation brush moves in a direction parallel to the optical axis of the light source lamp. A dust box receives dust removed from the rotation brush. The dust box includes an elongated container, which extends parallel to the rotation axis of the rotation brush, and a dust inlet, which opens toward the rotation brush to receive the dust from the rotation brush.