1. A charged particle beam irradiation method for processing and observing a workpiece by using a charged particle beam apparatus to apply a charged particle beam to a pixel that is a unit area for charged particle beam irradiation comprising the steps of: setting a process area based on an image obtained by observing the workpiece and determining the positions of representative points that form a contour of the set process area for each pixel with sub-pixel accuracy that is more accurate than a pixel accuracy, the position of each of the representative points being able to be set to either the center position of the pixel or a position displaced therefrom; and applying the charged particle beam, for the pixels that include the contour of the process area, to the positions of the representative points that form the contour with sub-pixel accuracy.