A semiconductor memory includes a composite floating structure where an insulation film is formed on a semiconductor substrate, Si-based quantum dots covered with an extremely thin Si oxide film is formed on the insulation film, silicide quantum dots covered with a high dielectric insulation film are formed on the extremely thin Si oxide film, and Si-based quantum dots covered with a high dielectric insulation film are formed on the high dielectric insulation film. Multivalued memory operations can be conducted at a high speed and with stability by applying a certain positive voltage to a gate electrode to accumulate electrons in the silicide quantum dots and by applying a certain negative voltage and weak light to the gate electrode to emit the electrons from the silicide quantum dots.