1. A method of measuring a total amount of latent image charge, the method comprising: scanning a sample with a charged particle beam, the sample having a charge distribution on a surface of the sample, the charge distribution being caused by forming an electrostatic latent image by exposure; measuring a state of a surface charge distribution on the sample based on a detection signal obtained by the scanning; obtaining an electric potential at a potential saddle point, the potential saddle point being formed by the surface charge distribution of the sample; obtaining an electrostatic latent image area, the electrostatic latent image being formed by the surface charge distribution of the sample; and performing a calculation using the electric potential at the potential saddle point and the electrostatic latent image area to measure a total amount of electric charge of the electrostatic latent image, wherein the electric potential at the potential saddle point is obtained based on detection of a primary electron signal, the primary electron signal being caused by a charged particle beam reversed before reaching the sample, the electrostatic latent image area is obtained based on detection of a secondary electron signal, the secondary electron signal being caused by a secondary electron generated when a charged particle beam reaches the sample, and the primary electron signal and the secondary electron signal are detected with one formation of a latent image.