An electrophotographic reproducing machine wherein an integrally formed dish-like housing for an exposure optical system is mounted onto a machine frame which accommodates therein a plurality of process units for reproducing and is opened upward, and a document glass plate is mounted at the top portion of the housing for the exposure optical system through glass mounting plates. The housing accommodates therein a slit plate for scanning of an exposure lamp, a projection lens, which is formed in its peripheral walls with vent holes for promoting ventilation within the machine and which has at its bottom portion a shield bottom wall serving as also a heat insulating plate. Guide rail surfaces for guiding movable mounts of the optical system are formed on both side walls of the housing opposite to each other. A projection lens mounting seat is integrally formed substantially at the center of the bottom wall of the housing, and a shield box raised upward from the bottom wall is integrally formed on the bottom wall at a position adjacent to the production lens mounting seat.