A laser beam spot shape correcting method includes a laser beam irradiating step of irradiating a concave mirror with a laser beam, an imaging step of imaging reflected light by a beam profiler, an image forming step of forming an XZ plane image or a YZ plane image from an XY plane image imaged in the imaging step, and a comparing step of comparing the image formed in the image forming step with an XZ plane image or a YZ plane image of an ideal laser beam. A phase pattern displayed on a display unit of a spatial light modulator is changed such that the XZ plane image or the YZ plane image formed in the image forming step coincides with the XZ plane image or the YZ plane image of the ideal laser beam.