An air passage opening/closing device for a vehicle air conditioner has a casing and a slide door. The slide door is movably disposed in the casing. Both sides of the slide door are guided by guide channels provided in the casing. The guide channels are formed by the casing and a separate guide member. The slide door can be assembled by mounting the slide door on a door-sliding surface of the casing, and mounting the separate guide member on the slide door.