An air conditioner includes an indoor unit to be installed in indoor space, an outdoor unit to be installed in outdoor space, the outdoor unit being separated by a building wall from the indoor space, a refrigerant pipe to connect the indoor unit with the outdoor, a ventilation pipe to connect the indoor unit to the outdoor space, a ventilation device to be arranged in the indoor space and to be connected to the ventilation pipe to ventilate indoor air in the indoor space to outdoor space or outdoor air in the outdoor space to the indoor space through the ventilation pipe, and a sleeve to be installed to pass through a hole formed at the wall to guide the refrigerant pipe and the ventilation pipe therethrough.