By a laser crystallization method, a crystalline semiconductor film in which grain boundaries are all in one direction is provided as well as a manufacturing method thereof. In crystallizing a semiconductor film formed over a substrate with linear laser light, a phase-shift mask in which trenches are formed in a stripe form is used. The stripe-form trenches formed in the phase-shift mask are formed so as to make a nearly perpendicular angle with a major axis direction of the linear laser light. CW laser light is used as the laser light, and a scanning direction of the laser light is nearly parallel to a direction of the stripe-form trenches (grooves). By changing luminance of the laser light periodically in the major axis direction, a crystal nucleation position in a semiconductor that is completely melted can be controlled.