1. A subject tracking device, comprising: a tracking zone setting unit that initially sets an area where a main subject is present within a captured image as a tracking zone; a tracking unit that detects a new area, where the main subject is present and which corresponds to the tracking zone set by the tracking zone setting unit, within a next captured image based upon an image output corresponding to the tracking zone, and sets the detected new area as a new tracking zone; and an arithmetic operation unit that determines through arithmetic operation image-capturing conditions based upon an image output corresponding to a central area that is within the set new tracking zone and smaller than the set new tracking zone.