A mechanism for always measuring the spatial intensity distribution of a laser beam and displacement of the optical axis of the laser beam is provided so that a measured signal is processed when the laser beam incident on a laser beam shaping optical element is out of a predetermined condition. The shape, diameter and incidence position of the laser beam incident on the laser beam shaping optical element are always kept in the predetermined condition by a spatial filter disposed at the position of a focal point of lenses forming a beam expander disposed in the optical axis, on the basis of a result of the signal processing.In this manner, silicon thin films uniform in crystallinity can be formed stably with a high yield on an insulating substrate which forms display panels of flat panel display devices.