PURPOSE: To obtain an element bringing a high-speed tracking function and a raster scanning correcting function under control in a raster output scanner.

CONSTITUTION: The element is an optical element 50 for deflecting a laser light beam in the raster output scanner(ROS) 10 of an electrostatic photographic printing machine. The element 50 includes a chamber containing ferroelectric liquid crystal material having a variable refractive index as the function of voltage impressed across the element itself. The ROS 10 includes a controller coupled with a power source in order to impress the voltage across the element 50 and performing high-speed scanning by the light beam by controlling the variable refractive index of the element 50 in relation to the rotation of a polyhedron 16, the matching error of a small surface 16a and the moving error of a belt.