PURPOSE: To provide both a laser scanning optical system for changing beam diameter on a scanning face and an electrophotographing device capable of properly controlling the beam diameter and luminous intensity of a laser beam according to the density of a picture element.

CONSTITUTION: An aperture is constituted of an aperture plate 3a wherein an aperture part 3b for regulating the diameter of a laser beam is formed, a shield plate 3e and switches 3f, 3g for measuring the position of the shield plate 3e. The shield plate 3e is moved to an opening position for opening the aperture part 3b to its full width by a solenoid 3d or to a shielding position for partially shielding the aperture part 3b. A control part controls the position of the shield plate 3e according to the preset density of picture elements. Thereby the beam diameter is changed.