Abstract: An air conditioner comprises an air-to-air heat exchanger; a fresh air supply duct, a fresh air by-pass duct, a used air exhaust duct, and a used air by-pass duct, the fresh air supply duct and the used air exhaust duct being respectively connected to the air-to-air heat exchanger, the fresh air supply duct having a part in common with the used air by-pass duct, and the used air exhaust duct having a part in common with the fresh air by-pass duct. Included are a condenser provided in the common part of the fresh air supply duct, and an evaporator provided in the common part of the used air exhaust duct, the condenser and the evaporator being parts of a heat pump. A blower is provided in the common part of the fresh air supply duct and is connected to the used air by-pass duct, and a separate blower is provided in the common part of the used air exhaust duct and is connected to the fresh air by-pass duct.