PURPOSE: To simply form a linear heat source having a periodic intensity distribution with accuracy by providing a member having at least two regions, different from each other in transmittivity to the radiation from the linear heat source, and passing the radiation from the linear heat source through the member.

CONSTITUTION: Ar laser light 6 projected from a laser light source 1 is passed through a lens 2 so that it may be focused at the position of a workpiece 5. The laser light passed through the lens is deflected to the workpiece by a mirror 3 for beam scanning. Driving the mirror as indicated by the arrow 7 allows the laser light 6 linearly scan the workpiece back and force. The resultant pseudo linear beam 8 is then passed through a substrate 4 for beam deformation 4. The beam deformation substrate 4 is formed by forming a striped Cr pattern on a glass substrate. The pseudo linear beam 8 is projected in the direction orthogonal to the surface with the stripe formed thereon. As a result, the beam passed through the glass substrate is applied to the workpiece 5 in a specific period.