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.