1. An image processing device comprising: a phase difference detection section detecting, on each of a plurality of parallax images obtained through shooting from a plurality of respective view point directions different from one another, a phase difference between the parallax images as a measured parallax magnitude, the measured parallax magnitude being a function of a focal length of an image-pickup lens and a subject distance; and a parallax control processing section performing an image processing for modifying the previously measured parallax magnitude, a parallax magnitude adjustment factor being based on the focal length of the image-pickup lens and the subject distance, wherein the parallax magnitude adjustment factor is set such that any changes in the measured parallax magnitude that are due to either a change in the focal length of the image-pickup lens or a change in the subject distance are eliminated.