Patent ID: 7649663

Claim:
An image forming apparatus, comprising: an optical beam scanning device capable of causing a luminous flux from a light source to scan a photoconductive surface of each of plural photoconductors in a main scanning direction, the optical beam scanning device comprising: a rotary deflector configured to reflect and deflect an incident luminous flux by plural reflection surfaces arrayed in a direction of rotation, and thereby cause the incident luminous flux to scan in the main scanning direction; a post-deflection optical system including plural optical devices and configured to guide the luminous flux reflected and deflected by each of the plural reflection surfaces in the rotary deflector, to a photoconductive surface of a photoconductor to which the luminous flux should be guided, a diffraction grating being formed on the surface of at least one of plural optical devices; and a housing that supports the plural optical devices; plural photoconductors on which an electrostatic latent image is formed by a luminous flux caused to scan it by the optical beam scanning device; a developing unit configured to develop the electrostatic latent image formed on the photoconductor; a belt that contacts the plural photoconductors; and a driving shaft that rotates the belt; and when a coefficient of linear expansion of a material of the housing is α H , a coefficient of linear expansion of a material which prescribes spacing between the plural photoconductors is α F , a coefficient of linear expansion of a material of a shaft which drives the belt is α S , a spacing between two photoconductors situated at both ends of the plural photoconductors arranged along the belt is L, a beam spacing in a sub scanning direction of luminous fluxes which should be guided to the two respective photoconductors is LB, and a temperature rise is t, the plural optical devices forming the optical beam scanning device are set to have an optical characteristic such that a quantity of change in the beam spacing LB is −(α H −2×α F +α S )×(L×t)+α H ×LB×t.