An object of the present invention is to prevent vibration of a sliding door within a guide groove through a fundamental review of the structures adopted in sliding door devices in the related art having a sliding door made to move along a direction intersecting an air flow path to control the flow rate or direction of air within the air flow path. The present invention is characterized in that a tension-applying means is disposed at a sliding unit of a sliding door, which is inserted at a guide groove formed on the inside of an air-conditioner case to apply tension between the sliding unit and the guide groove.