An optical beam scanning device including a plurality of optical elements that are arranged on a housing and give a plurality of scanning lines to a plurality of photoconductor drums with an interval L is provided. Optical characteristics are designed so that intervals of the beams in the sub-scanning direction are shifted only by -(.alpha.H-2.times..alpha.F+.alpha.S).times.L+.alpha.H.times.BP per unit temperature, where linear expansion coefficients of the housing, a frame for locating the photoconductor drums and a shaft, which is used for driving a belt or a recording medium carried on the belt on which images developed on the photoconductor are overlapped, are designated by .alpha.H, .alpha.F and .alpha.S, respectively, and an interval between light beams positions on surfaces to be scanned in a sub-scanning direction as an optical path reflection is expanded by mirrors from a deflecting surface to the surfaces to be scanned is designated by BP.

It is an object of the present invention to provide an optical beam scanning device and an image forming apparatus which suppress displacement of plural images formed by a different photoconductor in a sub-scanning direction by means of a simple structure even if temperature changes. The optical beam scanning device of the present invention has a single light deflecting device, a pre-deflection optical system that allows light beams from a plurality of light sources to enter the light deflecting device, and a post-deflection optical system including a first optical element for imaging reflected light beams from the light deflecting device on surfaces to be scanned for respective light beams. A second optical element, which has a positive or negative power opposite to a power of the first optical element in a sub-scanning direction, is provided in a position of the pre-deflection optical system where the light beams passes commonly and the light beams enter with distance in the sub-scanning direction.