An optical mechanism installed in an office machine, which has a machine base defining an internal space, and a paper-feeding mechanism arranged on the machine base, the paper-feeding mechanism having an inside receiving space and a bottom incident zone for the passing of light. The optical mechanism has an optical-path device formed of an image sensor and an optical module and mounted in the internal space inside the machine base, and a light-focusing structure mounted in the receiving space inside the paper-feeding mechanism above the incident zone for increasing optical path.