PURPOSE: To prevent any mildew from being even partly produced and a strange sound from being generated by uniformly applying mildew-proofing coating on the surface of a blade of a transverse flow fan with a specific film thickness.

CONSTITUTION: In a transverse fan having many blades 1, a mildew-proofing film is formed by applying a mildew-proofing coating where a mildew-proofing agent is dissolved in an organic high polymer component such as polyamide resin and drying them. Upon drying the mildew-proofing coating, a drying process is divided into a low temperature drying process and a high temperature drying process carried out thereafter. In the first low temperature drying process, viscosity of the mildew-proofing coating is improved while in the later high temperature drying process the coating is effectually dried. By setting the thickness of the mildew-proofing coating 1.4μm or greater, any mildew is prevented from being even partlty produced and any strange sound from being generated.