1. A document analysis system, comprising: a common character string calculation unit to extract character strings that are common between two texts and determining whether or not the two texts are to be set as calculation objects based on a number of the extracted character strings that are common; and a similarity degree calculation unit to calculate, when the two texts are the calculation objects determined by the common character string calculation unit, a similarity degree therebetween by using an approximation of a Kolmogorov complexity, and when the two texts are not the calculation objects, handling the similarity degree between the two texts as being dissimilar.