An outdoor unit for use in a separate type air conditioner which includes an outdoor unit and an indoor unit each having at least a compressor, a heat exchanger, and a expansion device, with one unit connected with the other by refrigerant-tubes so that they altogether form an air conditioning system. The outdoor unit comprises: a baseplate, a housing having a first air intake window (side air intake window) in one of its side walls, a second air intake window (rear air intake window) formed in the rear wall extending upright from the baseplate, and an air outlet window (front air outlet window) formed in the front wall; a partition board extending upright from the baseplate and portioning for the internal space of the housing into a fan compartment for accommodating the heat exchanger and a fan for supplying air to the heat exchanger; and a machinery compartment for accommodating the compressor. The outdoor unit is characterized in that: the heat exchanger mounted upright on the baseplate extends over the rear air intake window; the partition board is provided in front of the heat exchanger; the partition board is disposed in front of the heat exchanger by securing a peripheral frame of the partition board on the front end of a peripheral frame of the heat exchanger by screws applied from the rear of the heat exchanger.