A blank is formed of a sintered alloy by sintering a mixture of powdered materials including powdered aluminum, and at least one of powdered silicon, powdered magnesium, and powdered chromium. This blank, being made of the sintered body, can be molded into a shape close to the final shape of a rotary polygon mirror. For example, a mirror surface can be molded into a desired shape in advance. Therefore, the final product of the rotary polygon mirror can be obtained with ease by subjecting the blank to a simple finishing process by cutting.