1. A marker determination device in which feature points extracted from an image are arranged in a predetermined space, and portions of this predetermine space in which the number of said feature points is equal to or less than a predetermined number are defined as unique features, comprising: a feature storage that stores these unique features, and the feature points extracted from an existing mark; and a feature comparator that arranges the feature points extracted from said existing mark in said predetermined space, and determines that said existing mark can be used as a marker that is detectable from said image when the number of the feature points matching with said unique features is equal to or more than a predetermined number.