An aperture member including an opening having a predetermined shape and an image forming optical system having a short focal length are disposed at predetermined positions between a laser emitter and a sample, and a substantially square laser beam irradiation region is formed by reducing and forming an image of the opening shape on the sample. The aperture member and the image forming optical system are movable in an optical axis direction, and a size of substantially square laser beam irradiation region on the sample is variable. The size of the laser beam irradiation region is adjusted to a size of a unit attention region in an analysis target region on the sample, and a step width of scanning for moving the laser beam irradiation position is also adjusted to the size of the unit attention region.