In an air conditioning device having an air passage device that extends inside an instrument panel in a lateral direction of the body of a motor vehicle, the air passage device has a substantially L-shaped slit for attachment and detachment of an air filter, the slit being formed of a first slit opening downwardly and a second slit opening rearwardly of the vehicle body and a gap is provided between the second slit and a front end portion of a glove box. This permits the air filter of a square plate shape to be attached to and detached from the slit without interfering with the glove box, while being rotated slightly along the floor and toe board of the passenger compartment.