PURPOSE: To make the thickness of a fan casing thinner at portions where fan units are made smaller in diameter and reduce noises at portions where fan units are made larger in diameter by making the diameter of a cross-flow fan gradually smaller from its central part toward both right and left ends.

CONSTITUTION: A cross-flow fan 1 is constituted in such a way that, for example, three fan units 3 positioned at the central part of the cross-flow fan are formed to have a large diameter, fan units 4, 4 positioned at both ends of the fan units 3 to have a little smaller diameter than that of the fan units 3, and fan units 5, 5 positioned on both sides of the fan units 4 to have the smallest diameter. These fan units 3, 4, 5 are coaxially fixed to a rotary shaft 6 and driven for rotation by a motor 7 interconnected to the rotary shaft 6. In this relation, a casing 2 is so formed as to shape a curved surface, ranging from the central part 2a toward the end parts 2b, along the outer contour of the cross-flow fan 1. In the case where a cross-flow fan is constituted in such a manner, the end parts 2b of the casing 2 can be made smaller in their depth and, accordingly, the casing gives an impression of visually thin thickness.