An image forming apparatus includes a first support member, an image carrying member configured to carry a toner image thereon, and an optical writing apparatus connected to the first support member and configured to form an electrostatic latent image on the image carrying member. The optical writing apparatus includes an optical system having at least one optical element, a first casing configured to support the optical system, and a second casing configured to encase the first casing and to prevent intrusion of dust to the optical system.