The spot shape of a laser beam is detected by moving a table holding a detection substrate having a luminescent substance in an X direction and a Y direction with a laser beam focused by a lens applied to an area of the detection substrate where the luminescent substance is located. The intensity of light emitted from the luminescent substance is detected during the movement of the table, and a light intensity map is prepared indicating the light intensities detected in the light intensity detecting step at all of the X and Y coordinates of the luminescent substance. Spot shape images of the laser beam are formed according to a plurality of light intensity maps obtained by positioning the focusing lens at a plurality of detection positions changed in a Z direction perpendicular to a holding surface of the table.