Polygon mirror, optical deflector, optical scanning device, and image forming apparatus

A polygon mirror comprising a resin member, the resin member including:

• a first surface;
• a second surface opposite to the first surface;
• a through-hole extending from the first surface to the second surface;
• an inner side surface, which intersects with the first surface and the second surface, surrounding the through-hole;
• a plurality of outer side surfaces, each of which intersects with the first surface and the second surface on an opposite side of the inner side surface;
• and
• a die piece division line surrounding the through-hole and disposed on the second surface,
• wherein the die piece division line is closer to the outer side surfaces than the through-hole,
• wherein the second surface intersects with the individual outer side surfaces at intersection lines,
• wherein the intersection lines include a first intersection line and a second intersection line that intersects with the first intersection line at an intersection, which is a first corner portion, and
• wherein the die piece division line is provided closer to the through-hole than a first midpoint between the inner side surface and the first corner portion.