A light irradiation unit forms an electrostatic latent image pattern on a photosensitive member by irradiating the photosensitive member and a shielding unit with light. A detection unit detects, in a rotation direction of the photosensitive member, timing at which a surface potential of the photosensitive member changes depending on displacement of the electrostatic latent image pattern in an axial direction of the photosensitive member.