Optical scanning apparatus and image forming apparatus

A scanning optical apparatus comprising:

• a deflecting device configured to deflect a beam emitted from a light source, said deflecting device including a rotatable polygonal mirror configured to reflect the beam, a motor configured to rotate said rotatable polygonal mirror, a substrate carrying said motor and provided with a circuit configured to drive said motor, and a plurality of magnetometric sensors carried on said substrate and configured to detect rotation of said motor;
• and
• an optical box accommodating said deflecting device,
• wherein said substrate is provided with two holes configured to fasten said substrate to said optical box using screws,
• wherein said optical box is provided with two contact portion contacting a surface of said substrate opposed to said two holes in a state that said substrate is fastened to said optical box through said two holes, and
• wherein each of said magnetometric sensors is disposed at such a position that a distance from said magnetometric sensor to a line m connecting said two holes is smaller than a distance from said magnetometric sensor to a line n connecting the two positions on said substrate where the said two contact portions contact the surface of said substrate, or is disposed on the line m.