An irradiation pattern data generation method includes: a process for providing a design pattern having diagonal side portions that extend diagonally with respect to an X-axis direction and a Y-axis direction on an XY plane; a rectangular approximation process for approximating the design pattern to rectangles to generate a rectangular approximation pattern; a first correction process for shifting the side portions of the rectangular approximation pattern in the Y-axis direction to generate a first correction pattern; and a second correction process for enlarging the side portions of the first correction pattern in the X-axis direction and the Y-axis direction to generate an irradiation pattern.