1. A mass distribution measuring method of irradiating an ionizing beam toward a sample surface on a substrate, and detecting information including a mass-to-charge ratio and a detection position of generated ions, further comprising: changing a direction of irradiating the ionizing beam to the sample surface; acquiring a plurality of mass distribution images by irradiation from a plurality of incident directions; and synthesizing the plurality of mass distribution images, wherein the plurality of mass distribution images obtained by irradiating the ionizing beams from different directions are subjected to rotational transform before being synthesized, so that an absolute coordinate at each point on the sample is aligned with a coordinate of each point corresponding thereto on the mass distribution images.