A case of an air passage opening/closing device is formed with guide grooves comprised of case side seal surfaces and a guide wall surface, two ends of a door body part are arranged inside the guide grooves, a gear mechanism has a driven side gear which is provided at the door body part and a drive side gear which engages with the driven side gear, the driven side gear is formed to extend in parallel with a movement direction of a slide door at a portion of the door body part which is positioned further inside in the width direction from the guide grooves, and a portion of the door body part facing the guide wall surface or a portion of the guide wall surface facing the door body part is formed with interposed parts which are interposed between the door body part and the guide wall surface.