A composite system of a scanning electron microscope (SEM) and a focused ion beam apparatus (FIB) has an FIB lens barrel for irradiating a focused ion beam to an irradiating position on a sample surface and an SEM lens barrel for observing a machining state of the machined sample surface. The FIB lens barrel has an aperture defining at least one slit of a preselected pattern so that during irradiation of the sample surface with the focused ion beam, the aperture is irradiated by the focused ion beam with a width covering the slit to thereby machine the sample surface in the form of the preselected pattern of the slit.