An image forming apparatus includes an image bearing member having a photosensitive surface and an optical writing apparatus generating a laser beam modulated according to image data. The optical writing apparatus includes a housing, an optical deflector, an f-theta lens and a separator. The optical deflector is configured to rotate to deflect the laser beam. The f-theta lens is configured to correct the laser beam deflected by the optical deflector and to transmit a corrected laser beam towards the image bearing member. The separator is configured to separate an inside space of the housing into at least two sections including a first section in which the optical deflector is mounted and a second section in which the f-theta lens is mounted. The separator includes a heat resistant transparent plate disposed at a position of crossing passage of the laser beam between the optical deflector and the f-theta lens.