1. An image processor comprising: a perspective image generator that generates a first perspective image of a target viewed in a first direction, based on three-dimensional volume data of the target; a perspective image acquirer that acquires a second perspective image of the target viewed in a second direction; a corresponding point acquirer that acquires one or more pairs of first and second corresponding points respectively on the first and second perspective images; a first calculator that updates the position of the one or more first corresponding points on the first perspective image, based on a difference in position between the first and second perspective images; and a display image generator that generates a first display image, the first display image including the first and second perspective images, the one or more first corresponding points updated on the first perspective image, and the one or more second corresponding points on the second perspective image, wherein the perspective image generator changes the first direction into a third direction based on the difference in position between the first and second perspective images, and generates an updated one, viewed in the third direction, of the first perspective image.