Abstract: An air mixing damper, which is displaceable in a direction substantially perpendicular to an air flow direction, is disposed between an evaporator and a heater core in a casing that constitutes a vehicular air conditioner. In the air mixing damper, a plurality of ribs are formed so as to project on a side of a dividing wall that divides an air flow passage, and rack gears, which are enmeshed with a shaft, are formed on an inner circumferential surface on the opposite side thereof. In addition, when the air mixing damper is assembled in the casing, in a condition in which the ribs are in abutment against a sealing surface of the dividing wall, pinion gears of the shaft are enmeshed with the rack gears, whereby jostling of the air mixing damper is prevented.