A light beam scanning optical apparatus in which is capable of switching over the image density. The light beam scanning optical apparatus comprises a light source unit having at least three light emitting points, a control circuit block which selects and lights the light emitting points for switching over the image density at least either between different pages or within one page, an optical deflector for deflecting light beams emitted from the light emitting points, a scanning surface, a first optical system placed between the laser diode array and the optical deflector, and a second optical system placed between the optical deflector and the scanning surface. For switching over the image density, light emitting points corresponding to a specified image density are selected and lighted by the control circuit block. Thus, the switchover of image density in the direction of sub scanning is implemented not by mechanical operation of relatively slow response, but by electrical operation of quick response with selection of light emitting points.