Forming a semiconductor thin film comprises homogenizing the intensity distribution of light; carrying out the amplitude modulation such that the amplitude of light, of which the intensity distribution is homogenized, is increased in the direction of the relative motion of the light to the base layer; and projecting the amplitude-modulated light on the non-single crystal semiconductor layer formed on the base layer. Forming a semiconductor thin film comprises forming a non-single crystal semiconductor layer (10) on a base layer made of an insulating material; irradiating the semiconductor layer by light; and relatively moving the light to the base layer to crystallize the non-single crystal semiconductor layer. It includes homogenizing the intensity distribution of the light; carrying out the amplitude modulation such that the amplitude of the light, of which the intensity distribution is homogenized, is increased in the direction of the relative motion of the light to the base layer; projecting the amplitude-modulated light on the non-single crystal semiconductor layer formed on the base layer; providing a low temperature point in the light-irradiated surface and generating a start point of the crystal growth; and forming a single crystal region along the direction of the relative motion of the light to the base layer. An Independent claim is also included for a semiconductor thin film-forming apparatus comprising a light source (1) emitting the light; a homogenizer (3) for homogenizing the intensity distribution of the light emitted from the light source; a light absorption mask (5) for performing the amplitude modulation; a light projection optical system (6) for projecting the light that is amplitude-modulated on the non-single crystal semiconductor layer formed on the base layer; a phase shifter (8) or a mask having a light absorption dot for providing a low temperature point in the surface irradiated by the light; and a mechanism for relatively moving the light to the base layer.