In an optical beam scanning apparatus and an image forming apparatus equipped with the optical beam scanning apparatus of the present invention, plural optical members that irradiate scanning lines by laser beams (beams) emitted from plural light sources individually onto corresponding photoconductive drums are disposed within an optical unit housing and a housing cover, a fixing mechanism configured to fix part of a parallel plate provided within the optical unit housing and the housing cover is provided, and a twist angle adjusting mechanism configured to adjust an angle of twist of the parallel plate by rotating, in a specific direction, the parallel plate having the part thereof being fixed with the fixing mechanism is provided. According to the optical beam scanning apparatus and the image forming apparatus equipped with the optical beam scanning apparatus of the present invention, it is possible to adjust the curve and the inclination of the scanning line with ease and at high accuracy.