1. A text sentence comparison method comprising: converting a first text sentence and a second text sentence into a first R tree and a second R tree, respectively; calculating a distance between the first R tree and the second R tree on the basis of a distance between two R trees, which is defined at least in accordance with a condition of a mapping between vertexes of the two R trees; and calculating a distance between the first text sentence and the second text sentence on the basis of the calculated distance between the first R tree and the second R tree.