1. A scanning electron microscope comprising: an electron source; a first focusing lens for focusing a primary electron beam emitted from said electron source; an object lens diaphragm for removing an unnecessary region of the primary electron beam focused by said first focusing lens; a second focusing lens for focusing the primary electron beam that has passed through said object lens diaphragm; an object lens for focusing the primary electron beam focused by said second focusing lens on a sample; a deflecting means for the scanning of the sample by the primary electron beam; a secondary electron detector for detecting a secondary electron emitted from the sample due to electron beam irradiation; and an X-ray detector for detecting an X-ray emitted from the sample; wherein said scanning electron microscope has a function of focusing the primary electron beam on the sample by using said object lens when the magnification of an image to be scanned is higher than a preset value (high-magnification mode), wherein said scanning electron microscope has a function of focusing the primary electron beam on the sample by using said second focusing lens when the magnification of an image to be scanned is lower than a preset value (low-magnification mode), and wherein, in said low-magnification mode, either a first low-magnification mode in which the exciting current of said object lens is set to be a constant value independently of the accelerating voltage of the primary electron beam or a second low-magnification mode in which the exciting current of said object lens is changed as a function of the accelerating voltage of the primary electron beam is selected.