1. An imaging device comprising: an image sensor; an optical system for expanding a predetermined region of the image sensor and compressing a peripheral region thereof, the optical system being capable of changing the position of the optical system relative to the image sensor by rotating the optical system around a center which corresponds to an axis that is different from an optical axis; and a position changing unit for changing a position of the predetermined region with respect to the image sensor, wherein the position changing unit includes: a frame memory for storing a reference image; an object position detector for detecting a position of an object of interest based on the reference image and an image subsequent to the reference image; a movement value calculator for calculating a movement value of the optical system based on the position of the object; and a driver for moving the optical system based on the calculated movement value.