1. An image forming method in which an image is formed based on a comparison between a plurality of images obtained by applying an electron beam to a specimen at different tilt angles, the method comprising: obtaining a first transmission image with the electron beam applied in a first direction; obtaining a second transmission image with the electron beam applied in a direction that is different from the first direction, the second transmission image corresponding to an area on the specimen that is smaller than that corresponding to the first image, and the second transmission image being formed within a region that lies outside a peripheral region that is blurred due to tilting; and performing a correlation search in the first transmission image using the second transmission image.