Disclosed is a charged particle beam device, wherein multibeam secondary electron detectors (121a, 121b, 121c) and a single beam detector (140; 640) are provided, and under the control of a system control unit (135), an optical system control circuit (139) controls a lens and a beam selecting diaphragm (141) and switches the electrooptical conditions between those for multibeam mode and those for single beam mode, thereby one charged particle beam device can be operated as a multibeam charged particle device and a single beam charged particle device by switching. Thus, observation conditions are flexibly changed in accordance with an object to be observed, and a sample can be observed with a high accuracy and high efficiency.