PURPOSE: To effectively reduce a light spot diameter error in a sub scan corresponding direction based on the manufacturing/assembling error of an image- forming system in an optical scanner using an image-forming mirror diameter.

CONSTITUTION: This scanner is the optical scanner which image-forms luminous flux from light source devices 1, 2 by converging only in the sub scan corresponding direction by a cylindrical lens system 3 as a linear image extending longer in a main scan corresponding direction, and deflects it in constant angular velocity by an optical deflector 4 as deflected luminous flux, and converges it on a surface 7' to be scanned by an image-forming mirror system 6 as a light spot, and performs constant optical scan by the light spot, and the error of an image-forming position in the sub scan corresponding direction due to the manufacturing error/assembling error of an optical system can be corrected by performing displacement adjustment on the arranging position of the cylindrical lens system 3 in a direction of optical axis.