Patent ID: 7602540

Claim:
An optical deflector comprising: a rotary body that includes a polygon mirror; a rotation shaft that is fixed to the rotary body and rotates together with the rotary body; a bearing unit that rotatably supports the rotation shaft; and a board to which the bearing unit is fixed, wherein the following inequalities are satisfied: 1 2 ⁢ ⁢ π ⁢ ⁢ 3 ⁢ π ⁢ ⁢ Er 4 4 ⁢ ⁢ ml 3 ≤ 2 × P 60 ⁢ ⁢ and ⁢ ⁢ 0.8 ≤ t ≤ 1.8 , where P [revolution per minute] is number of revolutions of the rotary body, l [meter] is a distance between a center of gravity of the rotary body and the board, m [kilogram] is mass of the rotary body, r [meter] is a radius of the rotation shaft, E [pascal] is Young's modulus of the rotation shaft, and t [millimeter] is a thickness of the board.