1. An image processing device comprising: a light source configured to irradiate a light intensity-modulated at a modulation frequency to a target space; a light receiving element configured to receive the light reflected from an object in the target space and generate an electrical output corresponding to an intensity of the received light; and an image generator configured to generate a distance image having pixel values, each of which provides a distance value between the object and the image processing device, in accordance with a phase difference between the light emitted from said light source and the light received by said light receiving element, and a gray image having pixel values, each of which provides a gray value of the object, in accordance with the intensity of the received light.