A new method that makes possible a potential-profile measurement of an electrostatic latent image is proposed. The key technology is to detect a primary electron. When a surface potential is greater than acceleration voltage of the primary electron, the velocity becomes zero before the electron hits a sample. As a result, the primary electron reaches to a detector without reaching the sample. The potential distribution can be measured by detecting primary electrons while changing an applied voltage of a backside. This method is that the means of charging, exposing, and detecting are all incorporated in the same system, making real-time measurement possible.This system is being used to analyze the basis of an electrostatic latent image formed on a photoconductor. In order to confirm a phenomenon of reciprocity law failure, the latent-image depth was measured by changing the delay time when exposure was carried out a couple of times. As a result, the latent-image depth tends to be formed deep when the delay time becomes long.