In an image forming apparatus including a main assembly and a scanning optical apparatus which is inserted into the main assembly of the image forming apparatus in a substantially horizontal direction and is mounted to the apparatus main assembly, in order to fix the scanning optical apparatus to the apparatus main assembly in a state in which the scanning optical apparatus is inserted at a predetermined insertion position of the apparatus main assembly, a leaf spring member for urging an end of the scanning optical apparatus with respect to an insertion direction of the scanning optical apparatus is provided on the main assembly of the image forming apparatus or the scanning optical apparatus and a cushioning member press-contacting the leaf spring member between the main assembly of the image forming apparatus and the scanning optical apparatus is provided.