Abstract: A door mounting structure for an air conditioning system is disclosed. An axial end portion (121B) of a door (120) arranged in a case (110) is journaled rotatably on a bearing portion (111B) of the case (110). The forward end of the bearing portion (121B) is inserted into the bearing portion (111B) in such a manner as to be projected out of the case (110). In order to secure a predetermined clearance between the bearing portion (111B) and the axial end portion (121B), the bore of the bearing portion (111B) is larger than the diameter of the axial end portion (121B). A fixing member (130) is inserted into the portion of the secured clearance and fixed on the axial end portion (121B) or the bearing portion (111B). The insertability of the axial end portion into the air-conditioning case is improved and a defect such as an insertion failure is prevented.