A main lens comprises a second segment G5-2, a sixth grid G6 and an additional electrode GM disposed between the second segment G5-2 and sixth grid G6. A first-level constant voltage and a second-level constant voltage are applied to the second segment and sixth grid, respectively. A third-level voltage, which is at a level between the first level and the second level, is applied to the additional electrode. In accordance with the increase in deflection amount of an electric beam, the third-level voltage varies a value expressed by ((voltage to the additional electrode)null(voltage to the second segment))/((voltage to the sixth grid)null(voltage to the second segment)). An auxiliary lens comprising lenses including a third grid G3 and a first segment G5-1 has a focusing power decreasing in accordance with an increase in deflection amount of the electron beam.