PURPOSE: To obtain high heat resistance by hot-pressing a polymer alloy film comprising a polyimide and an alamide to form the shape of the dust cap and applying heat treatment to the dust cap at a temperature of a Tg of the polyimide.

CONSTITUTION: The dust cap for speaker is formed by using a polymer alloy film comprising a polyimide and an alamide as a major component. In this case, the blending ratio of the polyimide is selected to be 30% or over. Then, the polymer alloy comprising a polyimide and an alamide is processed into a film by casting, the film is hot-pressed to form the shape of the dust cap and heat-treated at a temperature more than a glass transition temperature (Tg) of the polyimide. That is, two-phase system of the alamide and polyimide at forming of the dust cap is formed into 1-phase system by a loss of hydrogen bonding of the alamide through the heat treatment, the both are molten and the dust cap having a far higher heat resistance than that of the polyimide single component is obtained.