A method includes dividing a single beam emitted from a coherent light source into at least two branch beams, and causing the branch beams to cross each other at a predetermined interference angle thereby generating interference pattern. The method also includes irradiating a target surface of a substrate with the interference pattern. The method also includes dividing the target surface of the substrate into a plurality of predetermined shapes, and repeating a first substep of irradiating each predetermined shape with every shot of the interference pattern, and a second substep of conveying the substrate in a stepwise manner such that the predetermined shapes overlap each other in the stepwise manner. The method also includes causing a line-to-line pitch of the interference fringes in one of the predetermined shapes to align with the line-to-line pitch of the interference fringes in a next predetermined shape upon repeating the first and second substeps.