A charged particle apparatus wherein a charged particle beam irradiates an object to be observed and secondary emissions such as secondary electrons thus emitted from a surface layer of the object are detected. A signal indicative of the detected secondary emissions is applied to a control grid of a cathode ray tube to modulate an electron beam irradiating a fluorescent screen thereof. A detector detects the electron beam intensity of the cathode ray tube and minimum and maximum or peak values of the detected electron beam intensity are compared with predetermined reference signals to obtain deviation signals indicative of the difference therebetween. The signal applied to the control grid of the cathode ray tube is controlled in accordance with the deviation signals.