To provide a top plate structure of a high-altitude installation type air conditioner capable of being thinned and achieving necessary rigidity, strength, vibrational property and the like including the behavior of a top plate in driving a fan.

In the air conditioner comprising a main body casing for storing a fan, a fan motor, a heat exchanger, a drain pump, a switch box and the like, and a top plate as a top face of the main body casing for suspending and supporting the fan, the fan motor, the heat exchanger, the drain pump, the switch box and the like, the thickness of the top plate is thinned to be less than a specific thickness, a plurality of reinforcement ribs are radially extended from an approximately central part of the top plate where the fan and the fan motor are supported, toward a radial outer peripheral part where the heat exchanger is supported, parts between the reinforcement ribs are flattened, and the rigidity, strength, deflection characteristics, vibrational property and the like can be improved to desired levels without mounting a number of auxiliary reinforcement ribs used in a conventional one, by properly adjusting and setting the number of the plurality of reinforcement ribs, and their sectional shape (restriction shape), depth and width.